Size:
Medium-sized flies up to 1/4-inch in length.

Color:
Most species are black, a few may be brown; body and wings covered with hairs making fly resemble a small moth.

Behavior:
Moth flies are associated most often with drains which is where the nickname, drain flies, originates. They are common inhabitants of sewage treatment facilities where they are known as sewage flies. Buildings located near sewage treatment facilities may experience regular infestations of flies flying from such facilities. The moth fly remembers a tiny moth in appearance and is a poor flier. It prefers dark areas and is commonly seen resting on walls near areas where it breeds. In commercial buildings, they are often seen in restrooms, mop closets, and boiler rooms, but they may appear in any area where water and organic matter may accumulate.

Moth flies breed in heavily decaying organic matter that remains fairly wet. In fact, this fly can breed in standing water that contains suitable amounts of organic materials, such as drain traps or sump pumps. In commercial kitchens, they are mostly associated with drains and very wet, poorly maintained areas such as underneath dishwasher machines. In homes, restrooms, and similar areas, they are associated with drains.

The key to controlling moth flies is finding the breeding sites and removing them by proper cleaning practices. A number of breeding sites can be found and eliminated while others may easily be overlooked. The inspection needs to focus on finding all sites where wet organic matter has accumulated and then removing that matter completely. Regular cleaning of drains using appropriate drain cleaners or bacterial drain cleaning products. Bleach and hot water do not sufficiently clean drains to control flies!
On occasion, drain line breaks under slabs or in crawlspaces can result in huge moth fly populations in homes, restaurants, or other commercial buildings. Drain breaks beneath a slab floor will require the expertise of a plumber to diagnose. If flies are found to be breeding under the slab, the slab will need to be broken open and the drain line repaired. If the wet, contaminated soil is not removed and replaced with fresh, dry soil, the moth fly problem will persist.