Studies of polypeptide synthesis thesis utilizing [35S]methionine, sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and “Western blot” analyses revealed that a cuticle-degrading protease with a chymoelastase specificity is the major protein product whenMetarhizium anisopliae produces infection structures on an artificial surface orin situ during penetration of host cuticle. Production of chymoelastase by infection structures is favored by levels of nutrients insufficient to induce catabolite repression. Addition of readily utilized nutrients, e.g., alanine, caused extensive growth on the host cuticle but repressed penetration and synthesis of chymoelastase. Thus, the pathogenic process involving infection-related morphogenesis and enzyme production occurs only when it is necessary for the pathogen to establish a nutritional relationship with the host. Studies with inhibitors (8-azaguanine and actinomycin D) suggest that control of enzyme production is at the level of transcription.