The gene SIT4 of S. cerevisiae, which codes for a protein structurally related to the catalytic subunit of mammalian protein phosphatase 2A, was disrupted in vitro. Analysis of glycogen synthase activity ratio in mutant haploid cells indicated that the enzyme was less active than in wild-type cells. On the contrary, glycogen phosphorylase alpha activity was much higher. The activation of glycogen synthase observed in wild-type cells after incubation with lithium ions was not detected in mutant cells. These results suggest that the product of gene SIT4, a putative protein phosphatase, could be involved in the control of glycogen metabolism in yeast cells.