The phospholipid composition of the stalked and swarmer cell types of the differentiating, Gram-negative bacterium Caulobacter crescentus was determined. The phospholipid composition of the stalked cell type was 86.5% phosphatidylglycerol, 10.4% lysylphosphatidylglycerol, and 3.0% cardiolipin; that of the swarmer cell type was 84.1, 11.4, and 4.4% respectively. Phosphatidylethanolamine, which is a major phospholipid component of most Gram-negative bacteria, was totally absent.