A comparative study of the pathogenicity of Candida albicans morphological mutant strains was made to relate chlamydospore production, germ tube formation, and proteolytic activity to candidiasis in mice. It was observed that the mycelium strains were more lethal than the yeast-like strains and that neither chlamydospore production, germ tube formation, nor nutritional requirements was related to pathogenicity in mice. Statistical analysis indicated that the culture media of the organisms and the strain and sex of the mice into which the cells were injected were important factors in the development of pathogenicity.