Drain flies, also known as moth flies, sink flies, filter flies, sewer flies, or sewer gnats, are a type of small fly that belong to the family Psychodidae. They are about the same size as gnats and fruit flies, ranging between 2mm and 5mm, but can be distinguished by their small, moth-like wings and round, fuzzy bodies. Drain flies lay eggs in the film that forms on the inside of pipes and drains when water pools and collects sewage. They can also lay eggs in wet mops and buckets, compost piles, storm drains with standing water, and wet areas around the garbage, birdbaths, or barns. The larvae of drain flies are sometimes considered beneficial in small numbers, as their strong jaws can cut through the hair and sludge waste in drains which might otherwise form clogs. However, unless this sludge layer is removed entirely, the adult flies will continue to find it and lay more eggs.

To get rid of drain flies, it is important to eliminate potential breeding sites. The breeding site must be located and thoroughly cleaned. This can include removing the drain cover, scraping some of the slime off the sides, and looking for thin, tube-like, drain-fly larvae. If the eggs are being laid deeper within the pipes, it may be necessary to use cleaning solutions to flush out the pipes. Preventing drain fly infestations requires clean pipes that are frequently flushed out either with water or cleaning solutions on a regular basis.