Analysis of protein degradation during the life cycle of Blastocladiella emersonii showed that (i) protein degradation is especially high during two phases of differentiation (sporulation, 12%/h and germination, 5%/h) in contrast with a much smaller degradation rate in the other phases (growth and zoospores, less than 1%/hr); (ii) protein degradation during germination in growth medium, as well as most of the germination process, is quantitatively unaffected by cycloheximide; (iii) a caseinolytic protease (pH optimum 5.5, apparent molecular weight 55,000 to 60,000) is present in extracts of zoospores and germinating cells; (iv) this protease activity is very low (perhaps absent) in extracts of late growth phase cells, but reappears during induced sporulation; (v) a different class of caseinolytic protease activity (pH optima 7 and 10; apparent molecular weight 25,000 to 30,000) is found in cellular extracts of late growth phase and early phases of sporulation; (vi) the latter class of enzyme activity is released into the medium during later phases of sporulation and is replaced in the cells by the former class. Speculations as to the roles of protein degradation in cell differentiation are discussed.