SUMMARYThe zygomyceteMucor mieheiis used by industry to produce an acid protease which coagulates milk in the process of making cheese. This study is the first step toward understanding the natural function of the enzyme. My studies of control of the acid protease ofM. mieheipresently show that in a defined medium a protein must be present for enzyme to to be produced. The enzyme activity was not derepressed in the absence of protein by limitation of nutrients. Cycloheximide blocked enzyme production. High levels of free amino acids (1%) appeared to repress enzyme activity. There appeared to be repression of enzyme activity in the presence of high levels of glucose, ammonium tartrate and magnesium sulfate. These observations were compared to the results of similar studies in other fungal systems.