Host specificity in certain groups of Chytridiales is of interest, not only because of the host-parasite relationship but also because it has been used as a primary basis for the separation of spp. Dictyomorpha dioica Mullins was capable of infecting 8 identified spp. of Achlya, 2 unidentified isolates of Achlya, and Thraustotheca clavata. All isolates tested of Saprolegnia, Isoachlya, Dictyuchus, and Calyptralegnia were resistant to the parasite. The absence of a uniform pattern of infection among isolates of the same host sp. suggests the existence of biological races of the parasite or of its host. It is probable that other members of the Olpidia-ceae, at present known to infect only 1 host, may be found capable of infecting a much wider range of organisms. The optimal temperature for infection and sporulation of D. dioica in Achlya flagellata was 25[degree]C. The age of the host culture at which miximal infection occurred was 24 hours.