The phospholipids of phototrophically-cultured Chlamydomonas reinhardtii 137+ have been quantitated at the cellular and thylakoid membrane levels. The alga contains three major phospholipids, phosphatidylglycerol (PG), phosphatidylethanolamine (PE), and phosphatidylcholine (PC), which together constitute about 14% of the cellular polar lipid complement. PG is the only phospholipid in an analytical fraction of thylakoid membrane isolated from the alga, and represents about 9% of the membrane's total polar lipid. Slightly more than half the cellular PG is localized in the photosynthetic lamellae. Hexadecanoic and octadecanoic fatty acids make up over 70% of the acyl groups esterified to these phospholipids, PG being the most highly unsaturated of the three. Differences in fatty acid profile between cellular and thylakoid PG, as well as the presence of PE and PC in the alga but not in its thylakoid, are indicative of a strict biogenetic regulation of the specific types and species of phospholipid associated with the photosynthetic membrane.