Mucor alternans (van Tieghem) was found to be a diphasic species in that it grew exclusively in the yeast-like budding phase under anaerobic conditions in the complex medium yeast extract – peptone – glucose broth and in tissue-culture medium 199. In the latter medium this growth form occurred also at 37C, at an initial pH of 7.2, and at glucose concentrations of 0.1 and 1.0%. The authors suggest that because of its synthetic nature the tissue-culture medium could be used with advantage in the study of nutritional requirements of dimorphic mucors.