Drain flies show up in bathrooms and kitchens and are often mistaken for moths. They are breeding inside your drains, in the gelatinous layer of organic sludge inside slow or infrequently used pipes. As long as that material is there, the flies keep coming. Remove it, and they are gone.
Identification
Adults are about 2 to 3 mm long, body and wings covered in tiny hairs that give a fuzzy appearance. The wings are broad and held tent-like over the abdomen at rest, which creates the moth-like silhouette. Color ranges from light gray to dark brown, often mottled.
Drain flies are poor fliers, doing short weak hops rather than flying across the room. You usually find them resting on the wall near the drain or on the wall around the sink.
The flies most confused with drain flies are fruit flies and fungus gnats. Fruit flies have distinctive red eyes and are associated with food. Fungus gnats are darker, thinner, and come from potting soil. Fuzzy, moth-like flies on the bathroom wall means drain flies.
Behavior and Habitat
Drain flies breed in the biofilm on the interior walls of drain pipes, a mix of decomposing organic matter, bacteria, hair, soap residue, and grease. Larvae live in and feed on this material before pupating and emerging as adults through the drain opening.
The most common sites are bathroom sink drains, shower and tub drains, floor drains in basements and laundry rooms, and sink overflow drains. Infrequently used drains are the highest-risk spots because nothing disrupts the film between uses.
The flies are active year-round indoors and do not need outdoor access. The life cycle runs one to three weeks depending on temperature. Females lay 30 to 100 eggs per batch in the sludge layer. Eggs hatch within two days. Adults live about two weeks.
Signs of an Infestation
The most obvious sign is seeing the flies themselves. They congregate near the drain they are breeding in, typically on the wall within a foot or two of the opening. Because they are weak fliers, they do not disperse much. Finding them in one bathroom sink area and not others points you directly to the source drain.
A simple test confirms the breeding site. Place a piece of clear tape, sticky side down, over the drain opening and leave it overnight. If drain fly adults emerge from that drain, some will stick to the tape. Run the tape test on each drain you suspect. This tells you which drains are actively infested and prioritizes where to focus treatment.
You will not typically see the larvae unless you remove the drain cover and look at the slime on the pipe walls with a flashlight.
Health and Property Risks
Drain flies do not bite and do not transmit disease to humans in normal residential exposure. They are primarily a nuisance. In rare cases, drain fly larvae have been associated with accidental intestinal myiasis, meaning they were ingested accidentally and survived briefly in the gut, but this is an uncommon medical oddity rather than a practical concern in a standard home.
They do not damage pipes or plumbing. The problem is the flies themselves appearing in bathrooms and kitchens, which people understandably find unpleasant, and which signals a sanitation issue in the drain system. In commercial food service settings, their presence is a health code violation.
Treatment Options
Removing the breeding material inside the drain requires mechanical action. Killing adults without addressing the source creates a continuous cycle.
Start by removing the drain cover and scrubbing the inside walls of the pipe with a stiff brush as deep as you can reach. Follow with hot water to flush debris. Then apply a biological enzyme drain cleaner at night, so it sits in the drain undisturbed. Repeat daily for five to seven days. These products use bacteria and enzymes to digest the buildup that brushing alone cannot reach.
What does not work: bleach, boiling water, or vinegar. They pass through too quickly to penetrate the sludge layer and the population rebuilds within days. Aerosol sprays kill adults but leave the source intact.
A professional uses a high-pressure drain flush, mechanical snaking for deeper buildup, and professional-grade enzyme or microbial treatment. For floor drains with standing organic material in the trap, a technician can clean the trap directly. Drain flies appearing from multiple locations in a building may point to a problem in the main drain stack, which requires a larger assessment. UV light traps near infested drains help reduce adult numbers during treatment, though they do not address the source.
Prevention
Keep drains active. Drain flies cannot establish in a drain that is used daily, because the water flow disrupts the film before it can accumulate enough to support larvae. A drain in a guest bathroom or a basement sink that goes weeks without use is the highest-risk spot.
For low-use drains, run water through them weekly and pour an enzyme cleaner in once or twice a month. Keep drain covers in place. A screen drain cover that allows water flow but reduces organic debris entering the pipe helps slow buildup.
In the kitchen, wipe down the drain opening area regularly and avoid letting grease or food debris sit in the drain. The same habits that prevent fruit flies from using kitchen drains also prevent drain flies.
Fix slow drains. A drain that barely moves water allows organic matter to accumulate against the pipe walls. A properly draining pipe stays cleaner. If a drain runs slow, snake it or use a drain cleaning product to restore flow.
What It Costs
In most cases, treating drain flies yourself costs $10 to $25 for a bottle of enzyme drain cleaner and some manual scrubbing. If you hire a professional for a standard residential drain fly job, expect to pay $100 to $200 for a single visit, which typically includes identifying all infested drains, mechanical cleaning, and enzyme treatment.
Persistent problems in homes with multiple affected drains or floor drains with standing organic buildup run $150 to $350. Commercial settings with grease traps and multiple floor drains cost more, often $300 to $600 or higher, and ongoing service contracts are common in those buildings because the conditions that create drain flies do not go away without regular maintenance.
When to Call a Professional
Call a professional if you have done the manual drain cleaning and enzyme treatment and flies are still present three weeks later. Either you missed a breeding drain, or the buildup in the pipe is deep enough that it needs professional-grade equipment to remove.
Also call if the flies are appearing from a floor drain with standing water in the trap, from a sump pit, or from multiple locations throughout the building. Those situations point to a systemic issue in the building’s drain system rather than a single clogged bathroom sink, and that requires a professional assessment to resolve properly.