SUMMARYSixty-four isolates ofChaetomiumwere grown under defined conditions, and their growth over a range of temperatures was noted. Five temperature groups were designated: mesophiles, transitional mesophiles, microthermophiles, thermotolerants, and thermophiles. Growth rates of species from each temperature group were compared and the maximum growth rates of thermophiles were found significantly greater than those of the nonthermophiles. Growth temperature data were used to support other salient features of the genus in establishing the synonymy ofC. verrucichaetawithC. abuenseandC. virginicumwithC. thermophilevar.coprophile.Growth-temperature data also support the synonomy ofC. erraticumwithC. gracile.