Adult flies are 3/8 inch long. Larvae are rarely seen. Cluster flies are dull and dark gray with golden hairs on the thorax. The abdomen has irregular light and dark patches. Cluster fly larvae are parasites of earthworms in the soil and never appear indoors.

Adult flies find their way inside in early to mid-autumn triggered by the arrival of colder weather and will cluster together, especially in attics, as they overwinter. Cluster flies may become active indoors in January as days get longer. They are attracted to daylight and will appear sluggish and noisy as they fly and buzz about in windows.

Cluster flies are nuisance pests that cause no direct damage. They seek shelter inside buildings for overwintering. Adults may fly more than a mile to a site that offers shelter. They will invade attics or wall voids on the sunny side of the house or building. Secondary pest issues may arise if large numbers of cluster flies die inside a structure. Scavenging warehouse and larder beetles may become numerous as they consume the carcasses of flies.