The tiny, wriggling worms you’re seeing in your drain are almost certainly drain fly larvae. These thin, grayish or dark-colored larvae live inside the slimy organic buildup that coats the inside of pipes, feeding on bacteria, fungi, and decomposing material. They’re harmless to humans but a reliable sign that a layer of gunky biofilm has built up in your plumbing, and they won’t go away until that buildup is physically removed.
What Drain Fly Larvae Look Like
Drain fly larvae are small, typically 4 to 10 millimeters long, with a slightly flattened, segmented body. They’re often translucent gray or dark brown, and some species have a faint stripe running down their back. Unlike earthworms or other household pests, they don’t have legs. You’ll usually spot them near the drain opening, clinging to the edges, or wriggling in standing water in showers, bathtubs, or floor drains.
The adults are equally distinctive: tiny, fuzzy, moth-like flies about 2 to 5 millimeters long, with broad leaf-shaped wings. If you’re seeing both the “worms” and these small flies hovering near sinks or bathroom walls, you’ve confirmed the source.
Why They’re in Your Drain
Drain fly larvae don’t just pass through your pipes. They live inside a sticky layer of organic slime that builds up on the walls of drainpipes over time. This biofilm is made of decomposing hair, soap residue, food particles, and bacteria. It’s the larvae’s food source and shelter, and it’s extremely difficult to wash away with water alone because it clings to pipe walls rather than sitting at the bottom.
Any drain that receives irregular use is especially vulnerable. Floor drains in basements, guest bathrooms, utility rooms, and laundry areas are common breeding sites because the water in the P-trap (the curved section of pipe designed to block sewer gases) evaporates when the drain isn’t used. A dry P-trap typically loses its water seal within two to three weeks, giving adult flies free access to lay eggs inside the pipe. Drains that do get regular use can still harbor larvae if enough organic material has accumulated on the pipe walls.
How Fast They Multiply
Drain flies complete their entire life cycle, from egg to adult, in about 21 to 27 days at room temperature. In warmer conditions (around 85°F), the cycle speeds up. Eggs hatch in less than two days. The larval stage, the “worm” phase you’re seeing, is the longest part: nine to 15 days at around 70°F, and as few as eight days in warmer environments. After that, they pupate for one to two days before emerging as adults.
Female drain flies can live up to seven days and lay eggs in batches directly on the biofilm inside your pipes. Males live only a few days. This fast turnover means a small problem can become a noticeable infestation within a few weeks if the breeding site isn’t addressed.
How to Confirm the Source
If you’re not sure which drain is the problem, you can run a simple overnight test. Place a strip of clear packing tape or adhesive tape sticky-side down over the drain opening before you go to bed. Leave it overnight. Any adult flies trying to emerge from the pipe will get stuck to the tape. Repeat this for a few nights on each suspect drain, since not every fly emerges every night. This helps you pinpoint exactly which drain is the breeding site before you start cleaning.
Check drains you might not think of right away: floor drains in the garage or basement, overflow holes in bathroom sinks, and rarely used shower stalls.
How to Get Rid of Them
The single most important thing to understand is that liquid treatments alone don’t work. Pouring bleach, vinegar, baking soda, or boiling water down the drain is ineffective because the larvae live protected inside that sticky biofilm layer on the pipe walls. Liquid just runs past them. Boiling water and harsh chemicals can also damage certain types of pipes.
Physical removal of the biofilm is the only reliable fix. Here’s what that looks like in practice:
- Scrub the drain opening. Use a stiff-bristled brush (a bottle brush or pipe brush works well) and a drain cleaner to scrub as far into the pipe as you can reach. The goal is to break up and remove the slimy buildup clinging to the pipe walls.
- Remove and clean drain components. Take out the sink stopper, shower drain cover, and drain trap. Clean each piece with a biodegradable cleaner or plain water before reinstalling them. Biofilm accumulates on these parts too.
- Use an enzymatic or foaming drain cleaner. After scrubbing, an enzymatic drain product or foaming treatment can reach deeper sections of pipe that your brush can’t. These products use biological agents to break down organic material over time. They’re a supplement to scrubbing, not a replacement.
- Clear any clogs. If the drain is slow or clogged, use a drain snake (also called a drain auger) to remove blockages. A clog creates a large reservoir of organic material that feeds larvae continuously.
You should see a significant drop in adults within a week or two after cleaning. If flies persist, the biofilm wasn’t fully removed, or there’s a secondary breeding site you haven’t found yet.
Keeping Them From Coming Back
Once you’ve cleared the infestation, prevention comes down to two things: keeping drains clean and keeping them sealed.
Run water through every drain in your home at least once a week, including floor drains and guest bathroom fixtures. This keeps the P-trap filled with water, which acts as a physical barrier between your living space and the sewer line. For drains that genuinely never get used, a waterless trap seal (a one-way silicone valve that drops into the drain body) creates a permanent barrier. It lets water flow down but blocks flies, odors, and sewer gas from coming back up.
If one drain in an area had drain flies, adjacent drains in the same room or zone likely have biofilm buildup too, even if you haven’t seen flies there yet. Clean and seal all of them. A periodic scrub of your most-used drains every few months, paired with an enzymatic cleaner, prevents the biofilm from building back up to a level that supports a new generation of larvae.