The formerly monotypic genus Agrenia is redescribed. The holarctic type species Isotoma bidenticulata Tullberg, 1876 is redescribed and a lectotype is designated. Six new species are described: polymorpha sp. n. (N. America), pilosa sp. n. (Japan), agilis sp. n. (N. America), cyanura sp. n. (N. America), riparia sp. n. (Holarctic) and atroviridis sp. n. (N. America). The genus is recognised by a set of apomorphous characters: Absence of sublobal hairs on the maxillary outer lobe, presence of dorsal tubercles on dens, a long subapical macrochaeta on inner side of dens, and presence of a tunica on unguis. Sexual polymorphism (epitoky) and cyclomorphosis are reported in several species. Neutralization of male secondary sexual characters are observed in some reproductive specimens. Distribution of the genus covers the arctic, boreal and montaneous parts of the northern hemisphere.