Cell-free extracts of the protozoan Tetrahymena pyriformis have been found to catalyze the following reactions: Phosphatidylethanolamine + L-Serine <==> Phosphatidylserine + Ethanolamine. Phosphatidylserine --> Phosphatidylethanolamine + CO(2) divided by Net: L-Serine --> Ethanolamine + CO(2). The biosynthesis of phosphatidylserine in this organism resembles that found in higher animals. In contrast, phosphatidylserine is formed in bacteria in a reaction between CDP-diglyceride and L-serine. A detailed description is presented of the enzyme that catalyzes the synthesis of phosphatidylserine in Tetrahymena by the exchange reaction, together with some properties of the phosphatidylserine decarboxylase in this organism.