How to Get Rid of Drain Flies for Good

To get rid of drain flies, find and scrub out the organic film they breed in, not just kill the adults. These fuzzy flies, also called moth flies or sewer flies, lay eggs in the slimy biofilm coating a drain. Spraying adults or flushing boiling water does nothing to that film, so a fresh batch hatches within one to two weeks. Remove the film and the flies stop coming. Related: sewer roaches.

Confirm they are actually drain flies

Drain flies are small, fuzzy, moth-shaped insects about 2 to 5 mm long (roughly 1/16 to 1/4 inch) from the family Psychodidae. They are gray to brown, hold their wings roof-like over the body, and fly in short weak hops rather than steady lines. You usually see them resting on walls near a sink, tub, or floor drain, not buzzing around food. That resting-near-the-drain behavior is the giveaway.

They go by several names: moth flies, sewer flies, filter flies, and sink flies. Do not confuse them with fruit flies (which swarm ripe produce and have red eyes) or fungus gnats (which come from overwatered houseplants). If the insects cluster on a wall next to a drain and barely fly, they are almost certainly drain flies.

Find the breeding drain with the tape test

Before you clean anything, find out which drain is the nursery. In a house with several sinks, tubs, and floor drains, only one or two are usually breeding, and cleaning the wrong one wastes effort. A strip of clear tape over each suspect drain overnight tells you exactly where the adults are emerging so you can target it.

1. Wait until evening, when adults emerge, and make sure each drain is dry on the surface.
2. Stretch a strip of clear packing tape sticky side down across each suspect drain opening. Leave a small gap at one edge so the drain can still breathe and flies are not blocked in.
3. Leave the tape in place overnight, or up to 24 hours.
4. Check the tape in the morning. Emerging adults get stuck to the sticky underside as they climb out.
5. The drain with flies on the tape is your breeding site. If several drains catch flies, treat every one.

Do not seal the drain completely for days at a time, and do not use this on a drain you actively need. The test is a one-night diagnostic, not a trap that solves the problem.

Remove the biofilm: the elimination order that works

Killing adults feels productive but changes nothing, because the eggs, larvae, and pupae are all safe inside the pipe. The fix is mechanical: physically scrape the gelatinous film off the drain walls and trap. Work in this order, because scrubbing beats pouring every time, and pouring chemicals first often just wastes money and can damage pipes.

1. Clear any standing clog first. If the drain is slow, the film is thicker and water cannot flush debris. Handle the blockage before you treat flies; our guide on how to clear the clog first walks through it by drain type.
2. Scrub the drain walls with a stiff brush. A long metal drain brush or a bent bottle brush is the single most effective tool. Push it several inches down, scrub the walls all the way around, and pull up the gray slime. Repeat until the brush comes back clean.
3. Break down what the brush missed with an enzyme drain cleaner. Bacterial or enzyme-based cleaners (bio-based products sold for drains) digest organic buildup over several hours. Apply at night when the drain sits unused so the enzymes have time to work.
4. Flush with hot water, then follow up. Hot water rinses loosened debris but gives only short-term control on its own. Use it as a rinse after scrubbing, not as the whole treatment.
5. Pull the trap for a stubborn drain. If flies keep coming after scrubbing, the P-trap or overflow may hold sludge the brush cannot reach. Removing and cleaning the trap clears it. If you are not comfortable pulling plumbing apart, find a licensed plumber.
6. Knock down remaining adults last. Once the film is gone, wipe up strays with a damp cloth or a light household spray. This step matters only after the biofilm is removed; do it first and you are back where you started in a week.

Method vs effectiveness: what actually removes the breeding film

Most popular drain-fly tips target the wrong stage. The table below rates each common method on whether it removes the biofilm where larvae live, versus just killing adults. Anything that only kills adults is a temporary fix, because the pipe keeps hatching new flies until the film is scrubbed out.

Keep them from coming back

Drain flies return whenever biofilm rebuilds, so prevention is really about not letting organic sludge accumulate. A little routine on the drains you rarely use is what keeps a bathroom or basement fly-free long term. Dry, moving, clean drains give females nowhere to lay eggs.

• Run water down rarely used sinks, tubs, and floor drains weekly so traps stay full and film stays thin.
• Scrub each drain with a brush once a month, and add it to your seasonal plumbing maintenance routine.
• Fix leaks under fixtures and clear standing water, since any damp organic surface can breed flies.
• Pour a little water into floor-drain and laundry traps that dry out, which also blocks odors.
• Clean the gunk from garbage disposals and overflow holes, common hidden breeding spots.

If flies persist after you have scrubbed every drain and the trap, the source may be outside the fixtures you can reach: a broken pipe under a slab, a leaking sewer line feeding wet soil, or a rarely used floor drain. Those cases often show up alongside a sewer smell in the house and are worth a professional look. A licensed plumber can camera the line and find the moisture source the brush never touched.

Frequently asked questions

Are drain flies harmful or do they bite?

Drain flies do not bite, sting, or transmit disease to people or pets. They are a nuisance and a sign of unsanitary buildup. Because larvae live in sewage and rotting organic sludge, adults can track bacteria onto surfaces, and heavy swarms may irritate the airways of sensitive people. Treat them as a sanitation warning, not a health emergency.

What causes drain flies to show up?

Drain flies show up when a slimy organic film, called biofilm, builds up inside a drain, trap, or overflow pipe. Females lay eggs in that gelatinous sludge and the larvae eat it. Slow drains, rarely used sinks, floor drains, and leaks under fixtures all let the film thicken. The flies are breeding inside your plumbing, not flying in from outside.

How long does it take to get rid of drain flies?

Expect about one to three weeks. Once you scrub out the biofilm and stop new eggs from hatching, existing adults die off within roughly 20 days, matching their natural lifespan. Clean the breeding drain thoroughly, then repeat every few days for at least three weeks to break the egg-to-adult cycle. Adults you see today are the last generation, not proof of failure.

Will drain flies go away on their own?

Rarely. As long as biofilm sits in the pipe, females keep laying eggs and a new batch hatches every one to two weeks, so the population sustains itself indefinitely. A drain that dries out completely for weeks may starve them, but any moisture and organic film restarts the cycle. Removing the breeding film is the only reliable way to make them leave.

Does bleach or vinegar kill drain flies?

Not reliably. Bleach and vinegar are liquids that sheet straight over the biofilm and rinse away without dislodging the sludge the larvae live in. University extension programs advise against pouring bleach down drains for this purpose because it is ineffective and can damage pipes and seals. Mechanical scrubbing that physically removes the film works far better than any poured chemical.

Last reviewed: August 2026.