Evidence is presented that demonstrates that phosphatidylserine synthase (CDPdiacylglycerol:L-serine O-phosphatidyltransferase, EC 2.7.8.8) from Saccharomyces cerevisiae is phosphorylated in vivo and in vitro by cAMP-dependent protein kinase. Phosphatidylserine synthase activity in cell extracts was reduced in the bcy1 mutant (which has high cAMP-dependent protein kinase activity) and elevated in the cyr1 mutant (which has low cAMP-dependent protein kinase activity) when compared with wild-type cells. The reduced phosphatidylserine synthase activity in the bcy1 mutant correlated with elevated levels of a phosphorylated form of the phosphatidylserine synthase Mr 23,000 subunit. The elevated phosphatidylserine synthase activity in the cyr1 mutant correlated with reduced levels of the phosphorylated form of the enzyme. There was negligible phosphorylation of the phosphatidylserine synthase Mr 23,000 subunit from stationary-phase cells. Pure phosphatidylserine synthase was phosphorylated by the cAMP-dependent protein kinase catalytic subunit, which resulted in a 60-70% reduction in phosphatidylserine synthase activity. The cAMP-dependent protein kinase catalytic subunit catalyzed the incorporation of 0.7 mol of phosphate per mol of phosphatidylserine synthase Mr 23,000 subunit. The specific cAMP-dependent protein kinase inhibitor prevented the phosphorylation of phosphatidylserine synthase and the inhibition of its activity by the catalytic subunit. Analysis of peptides derived from protease-treated labeled phosphatidylserine synthase showed only one labeled peptide. Phospho amino acid analysis of labeled phosphatidylserine synthase showed that the enzyme was phosphorylated at a serine residue.