What are drain flies?

Small flies in the moth fly family (Psychodidae) whose larvae live in the organic slime that coats the inside of drains, sewers, and septic systems. Adults rest on walls near sinks and showers.

No. Adult drain flies have no biting mouthparts and feed on liquids and nectar, if at all.

What do baby drain flies look like?

The larvae are thin, gray, legless worms about 1/4 to 3/8 inch long with a dark breathing tube at the tail. They live in the slime inside pipes, so you usually only see them if you pull the stopper or open the trap.

Drain flies vs fruit flies: how can I tell?

Drain flies are gray and fuzzy with broad wings held flat or roof-like, and they sit still on walls. Fruit flies are tan with red eyes, have clear wings, and hover around food.

Drain Flies: What They Are and Why They're in Your Bathroom

Q: Are drain flies harmful?

Not directly. They do not bite or sting, and they are not known to spread disease in homes. In large numbers their dead bodies can trigger allergies, and a heavy infestation can signal a broken pipe or sewer leak.

Q: What causes drain flies?

A buildup of hair, soap scum, and organic gunk inside a drain, overflow, or P-trap; rarely used drains that sit wet; and occasionally a cracked sewer line under the floor.

Drain flies are small, fuzzy, gray or tan flies, about 1/16 to 1/5 inch long, that breed in the slimy film inside sink, shower, and floor drains. They are harmless (they do not bite), but they mean organic gunk has built up in a pipe, and they keep coming back until you physically clean it out.

You will also hear them called moth flies, sewer gnats, filter flies, or simply "gnats in the bathroom." They belong to the family Psychodidae, and the most common species in US homes are Clogmia albipunctata and Psychoda species.

What do drain flies look like?

Shape: like a tiny moth. Wings are broad, oval, and densely hairy, held over the body like a roof or flat in a heart shape when resting.
Color: gray, tan, or nearly black; often a pale fringe on the wing edges.
Size: 1/16 to 1/5 inch.
Behavior: sit still on walls, mirrors, and the tub for long periods. When disturbed they hop and flutter a short distance instead of flying away. Squashed, they leave a powdery smudge.
Time of day: most active in the evening.

Baby drain flies (larvae)

The larvae are slender, legless, grayish worms up to about 3/8 inch long, with a darker head and a short breathing tube at the rear. They live inside the gelatinous biofilm on pipe walls, feeding on bacteria, hair, and soap residue, and they are very tolerant of hot water and low oxygen. Full development takes roughly one to three weeks depending on temperature.

Drain flies vs fruit flies

People often call drain flies "fruit flies in the bathroom." The difference matters because fruit flies are solved in the kitchen and drain flies only in the pipe.

	Drain fly	Fruit fly
Body	fuzzy, gray or tan	smooth, tan to brown
Eyes	dark	bright red
Wings	broad, hairy, roof- or heart-shaped	clear, narrow, held along the back
Behavior	sits still on walls, short hops	hovers and circles
Where	bathrooms, laundry, basement drains	kitchen, fruit, trash
Breeds in	slime in drains and sewage	fermenting food, sometimes drains

Side-by-side close-up of a fuzzy gray drain fly and a tan red-eyed fruit fly on a white countertop

So if you have "fruit flies in the bathroom" and no food around, they are almost certainly drain flies. See also gnats vs fruit flies and the general gnats page.

What causes drain flies?

Drain flies need standing moisture plus organic slime. Typical breeding spots:

Bathroom sink and shower drains, especially with hair and soap scum around the stopper.
Sink overflow holes, which hardly ever get cleaned.
Rarely used drains: a guest shower, basement floor drain, or laundry standpipe where the trap water stagnates.
Garbage disposals and kitchen drains.
AC condensate pans and lines, refrigerator drip pans.
Broken sewer lines under a slab or in a crawl space, leaking into soil. Suspect this if flies persist after every drain has been cleaned.
Outdoors: septic fields, clogged gutters, sewage treatment plants (where they are called filter flies).

Are drain flies harmful?

For most households, no. Drain flies do not bite, sting, or damage property, and there is no evidence that they spread disease to people in homes. Two caveats: large numbers of dead flies and their hairs can aggravate asthma or allergies, and a persistent infestation can be an early sign of a plumbing leak.

How to confirm which drain they come from

Do the tape test: put a strip of clear packing tape, sticky side down, loosely over each suspect drain at night (leave small gaps so air moves). Check in the morning. The drain with flies stuck to the tape is your source. Repeat for several nights because adults do not emerge every day.

Clear tape stuck over a sink drain with small flies caught on it — The tape test: flies stuck to the underside overnight mean that drain is the source.

How to get rid of drain flies, in brief

Find the source drain with the tape test.
Remove the stopper and scrub the pipe with a long, stiff drain brush; clean the overflow too.
Take apart and clean the P-trap if you can.
Use an enzyme or bacterial drain gel nightly for a week to digest what the brush missed.
Run water in unused drains weekly.

Pouring boiling water or bleach usually fails, because it slides past the slime instead of removing it. Full instructions are in how to get rid of drain flies.

Do drain flies bite?

No. Drain flies cannot bite or sting, and they do not feed on blood. If you are getting bitten in the bathroom, look for other causes, such as fleas from a pet or mosquitoes that came in through a window.

Why drain flies keep coming back

The usual reason is that the drain was rinsed, not scrubbed. Larvae live in a layer of slime that clings to the pipe walls and the underside of the stopper, and flushing hot water, bleach, or a quick squirt of drain cleaner leaves most of it in place. Another common reason is a second source: the overflow hole in a bathroom sink, a basement floor drain, or a shower nobody uses. Do the tape test on every drain in the house, not just the one where you see flies. Finally, remember that adults live about two weeks, so you will see a few for a while after a successful cleaning.

When to call a plumber or pro

If flies keep appearing after two weeks of cleaning every drain, the breeding site is likely hidden: a crack in a drain line under the floor, a leaking wax ring under the toilet, or wet debris in a crawl space. A plumber with a sewer camera is often more useful than a pest company here. Expect roughly $150 to $350 for a camera inspection (approximate).

Drain Flies