1. A conditional choline auxotroph was isolated. The growth of this mutant was markedly inhibited by the addition of a low concentration of myo-inositol to the culture medium. The growth inhibition was completely prevented by the addition of choline. 2. When this mutant cell was grown in the presence of myo-inositol, the intracellular level of phosphatidylethanolamine methyltransferase was very low, whereas in the cell grown in the absence of myo-inositol the enzyme level was normal. The addition of myo-inositol to the cell grown in the absence of myo-inositol caused a marked decrease in the methyltransferase level. 3. The reduction in phosphatidylethanolamine methyltransferase level by myo-inositol also occurred in various wild type strains of Saccharomyces cerevisiae. 4. The decreased methyltransferase activity was restored by the removal of myo-inositol from the culture medium. The restoration of the enzyme level was completely abolished by cycloheximide. Thus, it was shown that protein synthesis was involved in the change of phosphatidylethanolamine methyltransferase level by myo-inositol. 5. These results suggest that the synthesis of phosphatidylcholine from phosphatidylethanolamine by the methylation pathway is regulated by myo-inositol.