The essentials in 30 seconds
Drain flies, also called moth flies or drain gnats, are about 1/8 inch (3 mm) long, gray and fuzzy, and rest with their wings folded like a roof over the body. They fly weakly, in short hops, and stay close to where they breed: the slimy organic film inside drains. To get rid of them: find the drain by covering each one overnight with a cup coated in petroleum jelly, then scrub the film out with a stiff brush, clean the trap and run water through rarely used drains. Boiling water or bleach poured down the drain gives at best short-term relief because the film stays put. If flies keep appearing after every drain is clean, look for overflows, floor drains, drip pans or a broken pipe.
- Drain flies do not bite and cause no damage to the house.
- Insect sprays kill the adults only; the larvae keep hatching from the film.
- Large numbers usually mean a bigger source than a bathroom sink.
Drain fly, fruit fly or fungus gnat
Three small flies turn up indoors, and each points to a different source, so a quick look saves a lot of cleaning in the wrong place.
| Fly | Size and look | How it behaves | Where it breeds |
|---|---|---|---|
| Drain fly (moth fly) | About 1/8 in; gray, fuzzy, broad hairy wings held roof-like over the body | Weak flier, short hops; rests on walls near sinks and showers | Slimy film in drains, overflows, floor drains, drip pans |
| Fruit fly | About 1/8 in; yellowish to dark brown, usually red eyes | Hovers around fruit, bins and bottles | Ripe fruit, food-waste bins, bottles, sometimes drains |
| Fungus gnat | 1/16 to 1/8 in; dark, delicate, long legs and antennae, like a tiny mosquito | Gathers at windows; runs over potting soil | Damp compost in house plants |
If you have red-eyed flies around the fruit bowl, read our guide to fruit flies. Small black flies around house plants are fungus gnats: let the compost dry out between waterings.
A drain fly's wing veins run in parallel lines from base to tip, a pattern unique to this family.
Drain fly larvae and life cycle
Drain fly larvae are legless, gray, worm-like and about 3/8 inch long, with both ends a little darker and a breathing tube that lets them live in wet slime. You will rarely see them unless you scrape the inside of a drain.
| Stage | How long |
|---|---|
| Egg | Hatches in about 32 to 48 hours at 70 °F |
| Larva | 9 to 15 days in the slime |
| Pupa | 20 to 40 hours |
| Whole cycle | As little as 8 days; usually one to three weeks |
| Adult lifespan | About two weeks |
That short cycle explains why a drain left dirty produces a steady stream of flies, and why a few adults keep appearing for a week or two after you clean: they had already hatched.
How to find which drain the flies come from
Drain flies are weak fliers and usually rest close to where they hatched, so start in the room where you see most of them.
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01
List every drain in that room
Sink (plughole in British English), shower, tub, the overflow holes under the rim of the basin and tub, floor drains and any toilet that is rarely used.
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02
Set the cup test
Coat the inside of a clear plastic cup or jar with petroleum jelly or vegetable oil and turn it upside down over each drain opening. For overflows, tape a clear plastic bag over the hole.
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03
Leave it 24 hours
Flies emerging from the pipe stick to the inside of the cup.
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04
Read the cups
Any drain with flies in its cup is a breeding site. More than one drain may be involved.
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05
Confirm with a scrape
Lift the drain cover and scrape the side of the pipe with a dull knife or old spoon. Look for small gray larvae in the slime.
Clear packing tape laid over a drain opening at night, sticky side down, works the same way: check it in the morning for stuck flies.
How to get rid of drain flies by cleaning the drain
The key is removing the film. Killing adults alone changes little while larvae keep developing.
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01
Clear slow drains first
A slow drain keeps the film wet and thick. Snake it or remove the hair and debris so water flows freely. Our guide to unblocking a sink shows how.
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02
Scrub the pipe
Use a stiff, long-handled drain brush (or a brush-tipped snake) to scour the inside of the drain as far as it reaches. Work it up and down repeatedly.
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03
Clean the trap
Under a sink with a P-trap or J-trap, put a bucket underneath, unscrew the trap, empty it and scrub out the slime, then refit it and check for leaks.
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04
Do the overflow
Push a narrow bottle brush into the overflow hole of the basin and tub; slime builds up in that channel too.
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05
Rinse
Run plenty of hot tap water to flush loosened slime away and check that the drain flows freely.
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06
Use an enzyme cleaner
Bacterial or enzyme drain products digest the organic film. Follow the label. Regular use helps keep the film from building up again.
Other breeding spots when the drains are clean
If cups stay empty over every drain but flies keep coming, they are breeding in other wet organic material. Check these places.
| Place | What to look for | What to do |
|---|---|---|
| Basin and tub overflows | Slime in the overflow channel | Brush it out and flush with water |
| Floor drains in basements, laundry rooms or garages | Dry or slimy traps | Scrub, then pour water in regularly so the trap stays full |
| Toilets and tanks rarely used | Slime ring at the waterline | Clean and flush weekly |
| Drip pans under the refrigerator or an air handler | Standing water with sludge | Empty, scrub and dry; fix the cause of the standing water; see AC leaking water |
| Sump pit | Sludge around the pump | Clean the pit; see our sump pump guide |
| Leaking drain or sewer pipe | Damp patches, odor, flies near a wall, floor crack or in the crawl space | Call a plumber to inspect pipes in crawl spaces, wall voids and under the slab |
| Outside | Wet compost, dirty trash cans, rain barrels, wet mulch | Empty and clean; keep mulch from staying soggy |
Very large numbers of drain flies usually point to a source bigger than a few indoor drains, such as a broken pipe under a slab or, outdoors, a septic system or sewage works nearby. If the flies are far more numerous outside than inside, screens on windows and vents help, and so does moving outdoor lights away from doors or switching to yellow sodium-vapor or insect lights, which attract fewer flies.
A one-week plan to get rid of drain flies
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01
Day 1: identify and test
Confirm they are drain flies, then set a greased cup over every drain in the affected rooms, and tape bags over overflows.
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02
Day 2: read the traps
Note which drains caught flies. Scrape those drains and look for larvae.
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03
Day 2: clean
Clear slow drains, scrub the pipes and overflows, take apart and clean the sink traps, rinse with hot tap water.
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04
Day 3: enzyme treatment
Apply an enzyme drain cleaner at night to the drains you cleaned, following the label.
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05
Days 3 to 6: keep water moving
Run water in every shower, basin and floor drain you rarely use, empty drip pans and clean any slime ring in little-used toilets.
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06
Day 7: retest
Put the cups back over the drains for 24 hours. A few adults hatched before the clean may still be flying, but cups over clean drains should stay nearly empty.
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07
After day 7
If a drain still produces flies, clean it again and repeat the enzyme product. If every drain is clean and flies persist, look at the hidden sources above or call a plumber.
Are drain flies harmful
Drain flies do not bite people or pets and do not damage the house. They are mainly a nuisance, but they come from decaying organic matter and sewage, so they can carry microbes onto surfaces, which matters most in kitchens. In high numbers, fragments of dead flies can act as allergens and have been linked to asthma symptoms in sensitive people. Clean surfaces where they rest, especially food areas.
Drain flies: frequently asked questions
What causes drain flies?
A buildup of slimy organic film inside drains, overflows, floor drains or drip pans. Females lay their eggs in it and the larvae feed on it.
How do I get rid of drain flies fast?
Find the drain with an overnight cup test, scrub the inside with a stiff brush, clean the trap and use an enzyme drain cleaner. Adults fade within one to two weeks.
Does boiling water kill drain flies?
It gives at best short-term relief. It leaves the film where larvae live, so flies return.
Do drain flies bite?
No. They do not bite people or animals.
Are drain flies harmful?
Mostly a nuisance. They can carry microbes from drains to surfaces, and dead flies can trigger allergies in large numbers.
What do drain fly larvae look like?
Small gray, legless worms about 3/8 inch long, with darker ends, living in the slime on the inside of drains.
Why do I still have drain flies after cleaning the drain?
Adults already hatched live about two weeks, or another source is still active: an overflow, floor drain, drip pan or a leaking pipe.
Are drain flies the same as fruit flies?
No. Drain flies are gray and fuzzy with roof-shaped wings; fruit flies are brownish with red eyes and gather near fruit.
Sources
University guidance, checked on 27 October 2026.
- University of California IPM · Moth or drain flies, Pest Note 74167 ↗Identification, life cycle from 8 to 24 days, cup test for 24 hours, scraping drains, other breeding sites, boiling water and bleach myth, brushing, enzyme cleaners, insecticides discouraged, health notes.
- Ohio State University · Drain flies (moth flies) ↗Larva size, egg, larva and pupa durations, adult lifespan, breeding sites, glue-board container test, enzyme cleaners, bleach after caustic cleaners, no insecticides registered for drains, asthma.
- Penn State Extension · Moth flies in the home ↗Appearance, jar and plastic bag test for overflows, wire brush or steam, bacterial and enzyme products.
- Penn State Extension · Common urban pests ↗Fruit fly size, color and red eyes.
- University of California IPM · Fungus gnats ↗Fungus gnat size, appearance and behavior.
Still seeing fuzzy flies?
Put a greased cup over every drain tonight and let the traps tell you where to scrub tomorrow.