Those small, dark, worm-like creatures you’re finding in standing water, drains, or toilets are almost certainly fly larvae, not true maggots. The two most common culprits are drain fly larvae and rat-tailed maggots, both of which breed in stagnant, organic-rich water. They look alarming but are relatively easy to eliminate once you understand what’s feeding them.
What You’re Probably Looking At
True maggots (housefly larvae) are pale white or cream-colored and rarely show up in water. Dark or black larvae found in household water sources are nearly always one of two insects: drain flies or drone flies. Both lay eggs in water with high organic content, and their larvae thrive in the slimy buildup inside pipes, toilet tanks, and anywhere water sits undisturbed.
Drain Fly Larvae
Drain fly larvae are the most common find. They’re small, dark gray to black, and look like tiny flattened worms, typically under a centimeter long. They feed on the biofilm that coats the inside of drains: a layer of soap scum, algae, fungi, and bacteria that builds up over time. You’ll find them in sink drains, shower drains, floor drains, toilet bowls, and anywhere water collects beneath leaking pipes. They especially favor drains that aren’t used often, like a guest bathroom shower, where water sits in the trap for weeks.
The adult flies are tiny (2 to 4 mm), fuzzy, moth-like insects that rest on walls near the breeding site. If you’re seeing small gray or tan flies with a distinctive hairy appearance near your bathroom or kitchen, the larvae in your water are almost certainly theirs. These flies go by many names: drain flies, sewer flies, filter flies, moth flies.
Rat-Tailed Maggots
If the larvae are larger, cylindrical, and have what looks like a long tail, you’re dealing with rat-tailed maggots, the larvae of drone flies. The “tail” is actually a breathing tube that can stretch several times the length of the body. It reaches up to the water’s surface while the larva feeds below, letting it breathe without surfacing. These larvae are found in especially foul or heavily contaminated water: think neglected rain barrels, clogged outdoor drains, or backed-up sewage. They need water with a high concentration of decaying organic material to complete their development.
Where They Come From
Both types of larvae need two things: standing water and organic buildup. In a home, the most common breeding sites are sink drains, tub drains, toilet bowls, floor drains, and leaky shower pans. Any place where organic matter accumulates in dark, wet conditions is a potential nursery. Larvae can also enter your home directly from sewer pipes that back up or “burp” contents into your plumbing.
The key factor isn’t dirty water coming from your tap. It’s the sludge that accumulates inside your pipes over time. Even in a clean home, the interior surfaces of drain pipes develop a thin biological film made of bacteria, soap residue, hair, and other organic debris. That film is what the larvae eat. A drain can look perfectly clean from above while harboring a thick layer of biofilm just a few inches down.
Are They Harmful?
Drain fly larvae pose no direct health threat. They don’t bite, sting, or carry diseases in a household context. They’re a nuisance, not a hazard.
Rat-tailed maggots carry a small but real risk. If someone drinks heavily contaminated water containing young larvae or eggs, it can cause intestinal myiasis, a condition where the larvae survive temporarily in the digestive tract. This is uncommon and mostly documented in cases involving obviously fouled water or rotting fruit. Symptoms include abdominal discomfort. The larvae can’t reproduce inside the body and are eventually passed, but the experience is unpleasant. This is one more reason to avoid drinking from any water source that shows visible contamination or a foul smell.
How to Get Rid of Them
The larvae live inside the biofilm coating your pipes, and that biofilm is surprisingly resilient. Many common home remedies only provide partial, temporary results.
Bleach kills adult flies on contact but does not penetrate the biofilm where larvae are embedded. The biofilm has a protective structure that resists chemical penetration, so pouring bleach down the drain may clear the surface without reaching the actual breeding colony. Boiling water has a similar limitation: it loses temperature quickly as it travels through the pipe and only contacts a small fraction of the biofilm surface. Both methods reduce adult fly numbers for a day or two without solving the problem.
There are no EPA-registered pesticides specifically approved for treating drain fly larvae inside pipes. Even professional pest control operators can only treat the living space for adult flies, not the breeding site itself.
The most effective approach is mechanical cleaning. Use a stiff, long-handled brush to physically scrub the inside of the drain body, breaking up and removing the biofilm. This eliminates the food source and the larvae at the same time. For pipes you can’t reach with a brush, enzyme-based drain cleaners can help dissolve organic buildup over time, though they work slowly and need repeated applications. A single treatment from the drain opening rarely reaches the full extent of the biofilm deeper in the pipe system. For persistent infestations, a plumber can use specialized tools to clean the full interior length of the affected pipes.
Preventing Them From Coming Back
The single most effective prevention step is running water through every drain in your home regularly, at least once a week. Larvae colonize stagnant traps, so keeping water moving disrupts their breeding cycle. Pay special attention to guest bathrooms, basement floor drains, and any fixtures you don’t use daily.
Fix leaking pipes and drains promptly. Even a slow drip beneath a sink creates a moist patch of organic buildup that drain flies will find. Check under sinks, behind toilets, and around shower pans for hidden moisture. If you have floor drains in a garage or basement, make sure they aren’t accumulating standing water or debris.
Periodic drain cleaning, even when you don’t see larvae, prevents biofilm from building up to the point where it supports a breeding population. A quick brush scrub once a month in frequently used drains keeps the organic layer thin enough that larvae can’t establish themselves. For outdoor water features, rain barrels, or animal water troughs, covering them or changing the water frequently eliminates the stagnant conditions that attract egg-laying flies.