We evaluated the diagnostic value of antibody to Blastomyces dermatitidis A precipitin arcs for identifying B. dermatitidis cultures by the exoantigen technique with 88 isolates from the United States and Canada, 12 from Africa, and 1 each from India and Israel. In addition, we studied 190 mycelial-form isolates of other fungi which could be confused antigenically or morphologically with B. dermatitidis. Antigen extracts from all of the North American isolates, the Indian and Israeli isolates, and one of the African isolates reacted with the B. dermatitidis A antibody. The anti-A serum did not react with antigens from the other 11 African isolates or with antigens from any of the 190 heterologous fungi. Studies with unadsorbed and adsorbed antisera to selected North American and African isolates of B. dermatitidis showed that the North American isolates not only produced the distinct A antigen, but also formed another antigen that was common to all 11 of the antigenically less complex African isolates. The results of this study indicate that at least two serotypes of B. dermatitidis exist. All of the North American, Indian, and Israeli isolates and one of the African isolates contained the A antigen and were designated serotype 1. Most of the African isolates (11 of 12) constituted a second serotype that was deficient in A but showed an antigen designated K.