Abstract A membrane-bound phospholipase D (PLD) from Saccharomyces cerevisiae was solubilized from mitochondrial and plasma membranes and partially purified. The enzyme has an apparent molecular weight of approximately 60 kDa, is strictly Ca 2+-dependent and preferentially hydrolyses phosphatidylserine and phosphatidylethanolamine. Enzyme activity is significantly increased in membranes from cells grown on a non-fermentable carbon source. The Ca 2+-dependent PLD is distinct from PLD encoded by the SPO14/PLD1 gene. The 195 kDa SPO14/PLD1 gene product is specific for PtdCho, Ca 2+-independent and is activated by PIP 2. Furthermore, Pld1p has transphosphatidylation activity in the presence of ethanol and thus resembles the prototypic PLD activity found in mammalian cells and plants. In contrast, the Ca 2+-dependent PLD described here is not affected by PIP 2 and does not catalyze transphosphatidylation. Thus, the Ca 2+-dependent PLD characterized in this study appears to be a member of a novel family of phospholipases D.