You have sewer flies because somewhere in your home, there’s a buildup of slimy organic matter giving them a place to breed. These tiny, fuzzy insects (also called drain flies or moth flies) lay their eggs in the gelatinous film that coats the inside of pipes, and that film provides everything their larvae need to grow. The good news: once you find and remove the breeding site, the problem resolves quickly.
What Sewer Flies Actually Are
Sewer flies are small, moth-like insects about 2 to 3 millimeters long, roughly the size of a pencil eraser tip. Their wings and bodies are covered in fine hair, giving them a distinctly fuzzy appearance. When resting, they hold their wings in a roof-like shape over their body, which is why they’re sometimes mistaken for tiny moths. They’re weak, clumsy fliers. You’ll typically spot them sitting on walls or ceilings near the drain they emerged from rather than buzzing around the room.
If you’re not sure whether you’re dealing with sewer flies or something else, a few details help. Fruit flies have a compact, housefly-like shape and tend to hover around fruit bowls, trash cans, and recycling bins. Fungus gnats look more like tiny mosquitoes and stay close to potted plants with moist soil. Sewer flies are the fuzzy, moth-shaped ones that appear near drains and don’t stray far from them.
The Biofilm That Feeds Them
Inside your drain pipes, a thick layer of biological buildup clings to the pipe walls. This film is made of bacteria, food residues, grease, soap scum, and other organic debris. Research on domestic drain biofilms has found these layers are remarkably complex ecosystems, hosting bacteria, microscopic worms, and single-celled organisms. For sewer flies, this sludge is both food and nursery.
Female sewer flies lay their eggs directly in this film. The larvae hatch and feed on the organic matter, developing through four larval stages before entering a pupal stage and emerging as adults. The full cycle from egg to adult takes roughly 5 to 10 weeks depending on conditions, with warmer temperatures speeding things up. That means a single unnoticed breeding site can produce several generations of flies before you realize what’s happening.
Where They’re Breeding in Your Home
The most common source is a drain you don’t use often. Bathroom sink drains, shower drains, tub drains, floor drains in basements or laundry rooms, and toilets in guest bathrooms are all prime candidates. When a drain sits unused, no water flushes through to disturb the organic buildup, and the sludge layer thickens into an ideal breeding ground.
But drains aren’t the only possibility. Sewer flies will breed anywhere dark, damp organic matter accumulates. Leaky shower pans, air conditioning drip pans, refrigerator drip trays, and sump pump pits can all harbor them. In some cases, a cracked or damaged sewer line beneath the home creates a breeding site that’s harder to identify.
To pinpoint the source, try this: place a strip of clear tape over each suspect drain overnight, sticky side down, leaving enough of the drain uncovered for air to flow. Adults emerging from the pipe will get stuck to the tape. Check in the morning. Whichever drain catches flies is your breeding site.
Why Bleach and Boiling Water Don’t Work
Pouring bleach down the drain is one of the most common first attempts, and it almost never solves the problem. Bleach flows straight through the pipe without penetrating the thick biofilm clinging to the walls. It may kill a few larvae it contacts directly, but the eggs and remaining larvae embedded in the sludge survive. Boiling water has a similar limitation: it passes through too quickly to break down the buildup.
What actually works is physically removing the biofilm. A stiff drain brush or a pipe-cleaning brush, the kind that looks like a long bottle brush, is the most effective tool. Push it into the drain and scrub the interior walls of the pipe to strip away the organic coating. Follow up with an enzyme-based drain cleaner, which uses biological agents to digest the remaining organic matter over several hours. Repeat every few days until no new flies appear.
For floor drains or other drains that are harder to brush, enzyme drain treatments applied consistently over a week or two can break down the film on their own, though it takes longer than manual cleaning.
Are They Harmful?
Sewer flies don’t bite or sting, and for most households they’re a nuisance rather than a health threat. That said, they aren’t entirely harmless. A CDC-published study from a hospital in Germany found that moth flies breeding in the sewage system carried bacteria on their feet, picked up from the biofilm they emerged from. Of the flies analyzed, 41% carried drug-resistant bacteria. In a home setting the risk is lower than in a hospital, but the principle holds: these insects walk through sewage-associated sludge and then land on your walls, ceilings, and potentially your food prep surfaces.
In large numbers, the shed body hairs and fragments from dead flies can also trigger allergic reactions in sensitive individuals, particularly those with asthma.
Keeping Them From Coming Back
Prevention comes down to eliminating the conditions they need. Run water through every drain in your home at least once a week, including guest bathrooms, basement floor drains, and utility sinks. This flushes away early biofilm before it can thicken enough to support larvae.
Clean drain stoppers and the visible portion of your drain openings regularly, since hair, soap, and food particles trapped there accelerate buildup deeper in the pipe. If you have a drain you rarely use but can’t remove, consider covering it with a fine mesh screen to block adult flies from entering or exiting. Fix any leaks around shower pans, pipes, or air conditioning drain lines promptly, since even a small amount of standing moisture in a hidden space is enough to support a colony.
If you’ve cleaned every accessible drain and the flies persist, the breeding site may be in a section of pipe you can’t reach or in a damaged sewer line. At that point, a plumber with a pipe camera can inspect the line and identify cracks, offsets, or buildup further down the system that a brush can’t reach.