A variety of cultural conditions were examined to determine the relationship between pyruvate kinase isozyme patterns and morphology in Mucor racemosus. The results indicate that M. racemosus has two isozymes of pyruvate kinase, form A and form B, which are clearly separable on ion-exchange columns (diethylaminoethyl-cellulose). Addition of glucose to cultures growing on amino acids in air resulted in the induction of form A and the termination of form B synthesis. Cycloheximide added at the same time as glucose blocked the formation of form A but did not interfere with the termination of form B synthesis. Removal of glucose resulted in termination of form A synthesis and the induction of form B. Cycloheximide blocked the induction of form B and did not interfere with the termination of form A synthesis. The data show that the isozyme type is not directly related to morphology, but depends only on the presence or absence of glucose.