Drain Flies in the Bathroom
By Marcus Ellery, Pest Control Editor · Updated October 2026 · General information, not an inspection or a diagnosis
Small fuzzy flies with moth-shaped wings, resting on the wall beside a tub or sink, match the extension descriptions of drain flies. They breed in the slimy film that coats the inside of drains and traps. The extension fix is mechanical: find the drain they come from and scrub that film out. When scrubbing does not stop them, the sources point toward a leak or back-up in the sewer line instead.
What people usually describe
- Fuzzy gray or brown flies, smaller than a housefly, sitting still on bathroom walls or the shower surround.
- Wings held roof-like over the body, so they look like tiny moths.
- Weak, short flights of a few inches when disturbed.
- More of them after a trip away, or in a guest bathroom whose drains are rarely run.
- Flies that come back days after the drain has been cleaned.
What the extension sources say about them
Drain flies are one family of insects (Psychodidae), so this section covers where they breed and how fast, according to the cited extension services. If your insect does not match the description, see the broader bathroom guide linked below.
What they look like
Small, delicate and fuzzy, 1/16 to 1/4 inch long, pale brownish-gray to black, with hairy moth-like wings.
Source: UMD Extension
Where the larvae live
In the gelatinous film inside drains and traps, feeding on algae, bacteria and fungi. They also breed in sewer leaks, plant saucers, overflowing garbage and air-conditioner condensate pans.
Source: UMD Extension
How fast they build up
Egg to adult takes about 7 to 28 days and adults live about two weeks, so once breeding is established new flies emerge every day.
Source: UMD Extension
How many eggs
Females lay masses of 15 to 40 eggs on moist surfaces. At 70°F the eggs hatch in under two days and the larvae take 9 to 15 days to develop.
Source: UF/IFAS Extension
How to find out
- 1
Run the tape test on every drain
Cover each suspect drain with clear packing tape for 24 hours, then look for flies stuck to the underside. Test the sink, tub, shower, floor drain and any rarely used drain separately so you know which one is breeding. (UF/IFAS Extension)
- 2
Scrub the drain walls, not just the opening
Use a stiff or metal pipe brush to scrape the slime off the sides of the drain and overflow, then flush with hot water. Virginia Cooperative Extension describes this as the way to remove the breeding material. (Virginia Cooperative Extension)
- 3
Repeat the tape test a week later
Adults already flying will live about two weeks, so a few stragglers are expected. New flies on the tape after cleaning mean the source is still active. (UMD Extension)
- 4
Get a free or low-cost identification
Most state Cooperative Extension services, run through land-grant universities, will identify an insect from a specimen or a clear photo, often free or for a small fee. Some university labs, such as Cornell’s Insect Diagnostic Laboratory, take public submissions directly. Find your state’s land-grant university through the USDA NIFA land-grant university directory.
Try this while you wait
The extension method is a brush and hot water, with enzyme or bacterial drain cleaners as the alternative UMD Extension names. Packing tape runs the confirmation test. These are screening and interim tools only. None of them identifies the pest for you or stands in for a licensed professional or an extension identification. As an Amazon Associate we may earn from qualifying purchases. It never changes your price.
When to stop and call a licensed pro
- The tape keeps catching flies after two rounds of scrubbing. UMD Extension says breeding can occur in sewer leaks and back-ups, which takes a plumber with a camera, not a pest treatment.
- You smell sewage or see a damp patch on a floor or ceiling below a bathroom. Call a licensed plumber.
- Flies appear in a room with no drain at all. The source may be a condensate pan, a plant saucer or something hidden in a wall, and a pest professional can help locate it.
If it turns out you need treatment
National 2026 planning ranges from our calculators for the jobs this page most often leads to. Open one to price your own home. Not quotes, and not lab-tested.
| Job | Typical 2026 range |
|---|---|
| Exterminator cost calculator | $150 to $600 |
| Monthly Pest Control Plan cost calculator | $400 to $900 |
Local help
We do not recommend or rate pest control companies. Two neutral places to start:
- State pesticide regulatory agencies (NPIC directory): the state agencies that certify pesticide applicators. EPA advises confirming a pest control company’s license is current before hiring.
- USDA NIFA land-grant university directory: filter by state to reach your Cooperative Extension service, which can identify a specimen.
Advertising partner
If you would rather have a pro look, Angi passes your request to pest control companies in your area, who contact you with their own quotes. Angi is a paid partner of this site, not an endorsement of any company it sends you.
Free to ask, and you are never obliged to hire anyone. We earn a referral fee from Angi if you request a quote. That fee never touches your price or the numbers above.
Related identification guides
Frequently asked questions
Does pouring bleach or vinegar down the drain kill drain flies?
Virginia Cooperative Extension says bleach or vinegar down a drain is not very effective, bleach can damage pipes, and insecticides should never be used in drains. The film the larvae live in clings to the pipe walls, so liquid that runs past it leaves most of them alive.
Are drain flies harmful?
The extension sources cited here treat them as a nuisance and do not describe them as biting. What they can tell you is more useful: a steady supply of them means organic film or a leak somewhere in the plumbing.
Why do I only see them in the guest bathroom?
A drain that rarely runs keeps a moist, undisturbed film where larvae can develop. The tape test on that drain will usually confirm it, and running water through rarely used drains is part of keeping them clean.
Should I call a plumber or an exterminator?
If cleaning stops them, neither. If cleaning does not, the UMD Extension guidance on sewer leaks and back-ups makes a plumber the first call, since spraying adult flies does nothing to the breeding site.
Sources
This page summarizes what the agencies and university extension services listed above publish about this problem. It is general information, not an inspection, a pest identification or medical advice, and it cannot tell you what is in your home. Sources can change; check the linked page before acting on it. PestCost is not affiliated with any agency, university or association cited here. Spotted an error? Send a correction. Product links are Amazon affiliate links; the quote button is a paid Angi referral. Disclosure.