Twenty cases of blastomycosis have been confirmed in the RSA, 9 of which are presented for the first time. Patients came from all four provinces and the mean age was 40 years. Six cases were diagnosed between 1985 and 1987. Differences between strains of Blastomyces dermatitidis isolated in the RSA and in North America include morphological and cultural characteristics, mycelial-yeast conversion, antigenic structure, and compatibility in cross-mating experiments. The diagnosis of this disease can be made by direct examination of unstained specimens, by histological examination or by culture of the organism. Culture should be attempted in all cases for confirmation of microscopic findings.