Wingspan c. 8mm.
This species has been the subject of some classification confusion in the British Isles, with records of nigrella from the late 19th century being assigned to Cosmiotes freyerella. and more recent records named exiguella, later deemed synonymous with the present species.
It occurs in a number of locations throughout Britain and Ireland, and flies in at least two generations, with recent records being in May and September.
The larva feeds on annual meadow grass (Poa annua), mining inside the grass blades.