Extracts of phylogenetically diverse plans were surveyed for their ability to synthesize the following homoserine esters which are potential precursors for methionine and threonine synthesis in green plants:O-acetyl-,O-oxalyl-,O-succinyl-,O-malonyl-, andO-phosphohomoserine. Synthesis ofO-acylhomoserine esters was detected only inPisum sativumL. andLathyrus sativusL. Extracts ofP. sativum, a plant known to accumulateO-acetylhomoserine, catalyzed the specific synthesis of this ester from homoserine and acetyl-CoA. Extracts ofL. sativus, a plant known to accumulateO-oxalylhomoserine, catalyzed the specific synthesis of this ester from homoserine and oxalyl-CoA. None of the other plants surveyed, including representatives of the green algae, horsetails, gymnosperms, and angiosperms, catalyzed the synthesis of any of theO-acylhomoserine esters studied. In contrast, synthesis ofO-phosphohomoserine by the reaction catalyzed by homoserine kinase was demonstrated in extracts of all plants examined, including the two exceptional legumes.These results suggest that, among the five homoserine esters studied,O-phosphohomoserine is the major activated homoserine derivative in plants. Direct confirmation of the dominant physiological role ofO-phosphohomoserine in the synthesis of cystathionine in the transsulfuration pathway of methionine biosynthesis in plants has recently been provided (Datko, A. H., Giovanelli, J., and Mudd, S. H. 1974. J. Biol. Chem. 249: 1139-1155).