Studies of breeding relationships are integrated with the nomenclature of Agrocybe praecox. Type studies of potential synonyms are presented. Nomenclatorial and type studies indicate that Agrocybe molesta has been confused with A. praecox sensu lato. New field and microscopic characteristics are introduced which differentiate between A. molesta and the A. praecox group. A. molesta is redescribed to reduce confusion and a neotype is designated for this taxon. Four sibling species of Agrocybe praecox are identified and neotypes designated for A. praecox and A. gibberosa. A new species, Agrocybe montanus is described. / M.S.