The phospholipid cardiolipin, or diphosphatidylglycerol, is ubiquitous in eucaryotes. It is unique in structure, subcellular localization, and potential function. Because it is found predominantly in the mitochondrial inner membrane, it is an excellent marker for mitochondrial biogenesis. Cardiolipin is required for activity of several mitochondrial enzymes and possibly also for import of proteins into the mitochondrion. To understand the role of cardiolipin in these cellular events, it is necessary to characterize the enzymes of the cardiolipin pathway, as well as the genes that control the expression of these enzymes. To date, the structural genes encoding the cardiolipin biosynthetic enzymes have not been identified in any eucaryotic organism. However, considerable information is available regarding the regulation of this pathway in yeast. The activity and regulation of the first enzyme of the pathway, CDP-diacylglycerol:sn-glycerol-3-phosphate 3-phosphatidyltransferase (phosphatidylglycerophosphate (PGP) synthase, EC 2.7.8.5), has been characterized in two evolutionarily divergent yeasts, Saccharomyces cerevisiae and Schizosaccharomyces pombe. In contrast to the second and third enzymes of the pathway, this enzyme is highly regulated, both by cross-pathway control and by factors affecting mitochondrial development. PGP synthase from S. pombe (and cardiolipin synthase from S. cerevisiae) have been purified to homogeneity. The amino acid sequences of these enzymes, combined with the availability of the complete genome sequence from S. cerevisiae will simplify the cloning of these genes in the near future.