Fungus gnats are small, dark, short-lived flies, of the families Sciaridae, Diadocidiidae, Ditomyiidae, Keroplatidae, Bolitophilidae, and Mycetophilidae (order Diptera); they are sometimes placed in the superfamily Mycetophiloidea.
The larvae feed on plant roots and fungi, helping in the decomposition of organic matter. The adults are 2–5 mm long and are important pollinators of plants and carriers of mushroom spores. They carry diseases such as pythium on their feet. They may be quite annoying to humans as they fly into their faces, eyes, and noses.
Some fungus gnats are exceptionally hardy, being able to tolerate cold conditions through their possession of antifreeze proteins.