The biosynthesis of asparaginase II in Saccharomyces cerevisiae is sensitive to nitrogen catabolite repression. In cell cultures growing in complete ammonia medium, asparaginase II synthesis is repressed in the early exponential phase but becomes derepressed in the midexponential phase. When amino acids such as glutamine or asparagine replace ammonium ion in the growth medium, the enzyme remains repressed into the late exponential phase. The three nitrogen compounds permit a similar rate of cell growth and are assimilated at nearly the same rate. In the early exponential phase the internal amino acid pool is larger in cells growing with glutamine or asparagine than in cells growing with ammonium sulfate as the sole source of nitrogen.