Classification of the dermatophytes, a group of keratinophilic fungi, depends primarily on morphological criteria, but individual identification is often difficult. Many species have been created, based on what are often minor differences in structure or unacceptable characters such as isolation from different hosts or different clinical conditions. Biochemical techniques, used successfully in the differentiation of other micro-organisms, have had a limited application in the study of dermatophytes and were made the subject of special investigation. 22 species were grown on media containing 30 different carbon or nitrogen sources. It was found that both geophilic and zoophilic species had similar nutritional patterns, but for some, e. g. Microsporumýypseuxn and Trhyton mentagrophytes, nutritional and biochemical tests are of little taxonomic value. Ability to hydrolyse urea allows differentiation of T. mentagrophytes from T. rubrum and T. erinacei, and T. soudanense from M. ferrugineum. Other nutritional differences which may be used to separate species are described. The results obtained are complementary to the classification of dermatophytes based on morphological criteria but do support the separation of certain species, e.. a. M. equinum, hitherto regarded as variants of another species. Serological investigations are described and five groups have been established, based on the presence or absence of common antigens. The results show that the genus Microsporum, with the exception of M. gypseum and M. persicolor, is serologically distinct from the genus Trichophyton. M. gypseum, T. ajelloi and M. persicolor are serologically related, however, having common antigens. The genus Trichophyton was found to contain two serological sub-groups: (1) T. mentagrophytes-T. rubrum-T. schoenleinii, and (2) T. quinckeanum- T. soudanense-T. erinacei. T. tonsurans has close relationships with most other Trichophyton species. T. violaceum and Epidermophyton floccosum had few antigens in common with any other species. The two saprophytic species, T. terrestre and M. cookei, appeared to be serologically unrelated to other members of the respective genera.