Abstract: Yeast mutants lacking proteinase A were isolated. One of these mutants (Hb I) is characterized in detail. The mutation called pra1 segregates 2:2 in meiotic tetrads indicating a single gene mutation. No anti-(proteinase A) cross-reacting material can be detected. Diploids heterozygous for pra1 show gene dosage. Thus, it appears that PRA1 might be the structural gene for proteinase A. Results obtained with this mutant show that proteinaseA is not a vital component of the vegetative cell cycle. The mutant exhibits normal mitotic growth under rich and poor growth conditions and shows normal mating. Enzymes subject to carbon catabolite inactivation and inactivation of NADP-dependent glutamate dehydrogenase, processes which were proposed to be of proteolytic nature, are not affected by the absence of proteinase A. However, protein degradation under sporulation conditions is about 30% reduced in proteinase A mutant cells. The differentiation process of sporulation is also disturbed leading to a 40% reduced sporulation frequency in mutant cells.