Drain flies are the small, fuzzy, moth-like flies that sit motionless on the wall near a sink or tub and flutter weakly when disturbed. They're harmless, they're persistent, and they're almost always breeding within a few feet of where you're seeing them.

What they're actually breeding in

Not the water — the film. A slick organic layer builds up on the inside walls of a drain pipe, and drain fly larvae live in it. This is why pouring boiling water or bleach down the drain rarely fixes anything: liquid runs straight through and down, barely touching the film clinging to the pipe walls.

Find the right drain first

Homes have more drains than people remember. Before you scrub, work out which one is producing: dry the drain, then lay a strip of clear tape sticky-side-down across the opening overnight, leaving a small gap for air. Check it in the morning — flies stuck to the underside mean that drain is the source. Repeat for each candidate.

Don't skip:

  • Floor drains in basements, laundry rooms, and garages.
  • The overflow hole in a sink or tub — a very common hiding spot.
  • Guest or basement bathrooms that go weeks without use.
  • Condensate drain lines from air conditioning units.
  • The drain pan underneath a refrigerator.

Clear the film

The fix is mechanical first, then biological:

  1. Scrub. Use a stiff, long drain brush and work the inside walls of the pipe as far down as it reaches. This is the step that actually matters.
  2. Follow with an enzyme or bacterial drain cleaner. These digest the remaining organic film rather than flushing past it. Chemical drain openers are built for clogs, not biofilm, and generally won't solve this.
  3. Repeat. Once won't do it. Plan on treating every few days for two weeks.

For a drain that's rarely used, run water through it weekly afterward. A dry trap lets flies and sewer gas come up from below.

Handle the adults while you wait

Larvae already in the pipe will keep emerging for a week or two after you start treating. Hanging fly strips near the affected drain catches those adults during the gap, so the room stays tolerable while the treatment does its work.

If they keep coming back

Persistent drain flies after thorough cleaning usually mean one of two things: a drain you haven't found yet, or a cracked or leaking pipe letting waste seep into the space beneath a floor or behind a wall. If you've cleared every drain and they still return, that's the point to have the plumbing looked at.