They're tiny, fuzzy, and oddly slow. Drain flies — also called moth flies or sink flies — drift up from a drain, settle on the wall above it, and barely move when you wave at them. Swatting them accomplishes nothing: for every adult on the tile, eggs and larvae live a few inches down the pipe, feeding on the gel-like film coating the drain walls. The fix isn't killing flies. It's evicting them, by removing the film they can't live without.
Three different gnats haunt kitchens and bathrooms, and each one points at a different problem. Drain flies are about 1/8 inch, fuzzy-bodied, with broad wings held like a little roof — they really do look like miniature moths, and they're weak fliers that make short hops. Fruit flies are smaller, tan, with red eyes, and they hover over fruit bowls, recycling bins, and empty bottles; they have nothing to do with your drains. Fungus gnats are darker and more mosquito-shaped, with long legs, and they breed in the wet soil of houseplants. If your flies have the fuzzy moth look and cluster near a sink, tub, or shower, you're in the right article. If they pour out of a slow drain as well, that drain needs attention too — the same buildup slows water, as covered in our guide to a sink draining slowly.
A house has half a dozen drains, and the flies usually breed in just one or two of them. Guessing wastes days. The tape test takes one night and tells you exactly where to work.
Dry the area. In the evening, wipe the sink, tub, or shower drain and the surface around it completely dry. The tape has to stick, and the flies have to be unable to slip out around it.
Tape over the drain. Cover the drain opening with clear packing tape or painter's tape, sticky side down, leaving small gaps at the edges. The gaps matter: a fully sealed drain traps everything, and you want flies to try to exit and get caught.
Check in the morning. Flies stuck to the tape mean that drain is a breeding site. No flies doesn't clear it entirely — repeat a second night before ruling it out. Test every suspect drain: bathroom sinks, the tub, shower, kitchen sink, laundry and floor drains.
Don't forget the drains nobody thinks of. Floor drains in basements and laundry rooms are classic breeding sites because they sit wet and undisturbed. Overflow openings grow the same film, and so does the condensate drip pan under a fridge or AC unit.
Here's the part most advice gets wrong. Pouring things down the drain — boiling water, baking soda, vinegar, bleach — rinses the middle of the pipe while the biofilm clings to the walls, barely touched. Larvae live inside that film, protected. What removes it is friction: a brush, worked around the pipe walls, physically stripping the layer off.
Open the drain. Remove the stopper, strainer, or drain cover. On a sink, unscrew the stopper from below if it won't lift out. You'll probably see the film already — a dark, jelly-like coating.
Scrub hard and deep. Push a stiff drain brush or bottle brush into the pipe and work it up, down, and around, as far down as it reaches. Scrub the overflow channel too, with a smaller brush or an old toothbrush at the opening. The goal is to break the film loose from every surface you can touch.
Flush with hot water. Run hot tap water for a minute or two to wash the loosened sludge down. Very hot water helps here because it carries the debris you already freed — it's the rinse, not the treatment.
Follow with a bio-enzyme cleaner. Pour a bio-enzymatic drain cleaner — the kind sold for drain maintenance, not a chemical opener — down the drain at night, per the label. The enzymes digest the organic film your brush couldn't reach, deeper in the trap. Repeat nightly for about a week: one application rarely eats through the whole layer.
Repeat the tape test. After a few nights, tape the drain again. Declining counts mean you're winning. The full life cycle runs roughly one to two weeks, so judge the result at the end of the second week, not on day three.
Bleach fails on contact time. It rushes past the biofilm in seconds, kills the surface microbes it touches, and leaves the bulk of the film — and the larvae embedded in it — intact. It's also rough on pipes and septic systems used repeatedly. Boiling water is better but incomplete: it can kill exposed larvae and loosen grease near the top of the trap, which makes it a decent supporting move and a poor solo act. Both approaches share the same flaw — they treat the water path, not the pipe wall. Mechanical scrubbing plus enzymes is the combination that works because the brush removes what chemistry can't reach. While you're working on drains anyway, the habits in our guide to unclogging a sink without harsh chemicals keep the same pipes from building this kind of sludge back up.
Two things regrow an infestation: fresh film and standing water. Slow the film by running hot water through little-used drains weekly and brush-scrubbing problem drains every month or two — a two-minute job once you've done it the first time. Fix the water side too: a slow leak under a sink grows its own film on the outside of the plumbing, where flies breed just as happily, so check for dampness under every sink. Empty and rinse the fridge drip pan and AC condensate pan if your tape test pointed there. Bathtub drains grow the same film under their stoppers, which is why this scrub routine mirrors the one used to unclog a bathtub drain.
Spraying the adults. Bug spray kills today's fliers and impresses no one underground; the next generation hatches within days. One-and-done treatment. A single scrub night knocks the population down and then it rebounds — the nightly enzyme week is what finishes it. Testing one drain and stopping. Treat every drain the tape test flags, including the floor drain you forgot you owned. Misidentifying the fly. Plenty of effort gets wasted on drains when the real source is fruit flies over the compost bin. Confirm with the fuzzy wings and the tape test first.
Drain flies are a cleaning problem wearing a pest costume. Find the breeding drain with tape, strip the biofilm off the pipe walls with a brush, let a bio-enzyme cleaner digest the rest over several nights, and fix any leak or standing water feeding it. No poison, no exterminator, and the drain usually runs a little faster as a bonus.
They don't bite and they aren't known to spread disease to people. They're a nuisance and a sign of organic buildup in a drain — the film they'd been living in is the actual hygiene issue, and that's what the scrubbing removes.
Plan on about a week to two. Adults disappear within days once the film is scrubbed out, but eggs already laid can keep hatching for several days, which is why the enzyme treatment runs nightly for a week. If numbers aren't dropping by the end of week two, there's another source drain you haven't found.
It kills exposed flies and larvae it actually reaches and loosens grease, but it flows past the biofilm on the pipe walls without removing it. Use hot water as the rinse after scrubbing, not as the treatment itself.
Almost always one of three reasons: a second breeding drain was missed (floor drains are the usual suspect), the scrubbing didn't strip the film fully, or a hidden leak is keeping organic matter wet somewhere — under a sink, behind a tub overflow, or in a condensate pan. Re-run the tape test on every drain.
Call a plumber if flies pour out of multiple drains at once, you smell sewage, or the infestation survives two honest weeks of scrubbing — that pattern can mean a cracked sewer line or a dried-out trap letting sewer pests in. The sewage smell on its own — flies or no flies — usually traces to that same dry or faulty trap, and our guide to a sewer smell in the bathroom walks through pinning it down. Pest control can identify the insect if you're unsure what you're looking at, but the fix is still removing the breeding film.