SUMMARYCultural features and morphology of 11 isolates of the hyphomyceteHirsutella thompsonii, an important pathogen of various eriophyoid mites, are described and illustrated. Three morphologically distinct groups ofH. thompsoniiwere distinguished and accordingly three varieties are proposed:H. thompsoniivar.thompsonii, var.vinaceaand var.synnematosa.The varietyvinaceais characterized by vinaceous colonies and a different host,Acalitus vaccinii.The strains accommodated in the var.synnematosawere isolated fromEriophyesspp. and related genera. In fresh culture they produce cream-colored cylindrical synnemata, bearing two kinds of phialides: (1) single phialides with solitary conidia and (2) densely crowded phialides with catenulate conidia. In all three varieties two different conidiogenous structures are formed: (1) solitary often proliferating phialides producing one or more globose, verrucose conidia and (2) polyblastic conidiogenous cells with smooth-walled, subglobose to ellipsoidal conidia. AlthoughH. thompsoniidiffers in some morphological characters from other species in the genus, its maintenance inHirsutellais proposed.