The major lipids of Vibrio costicola membranes and whole cells are phosphatidyl glycerol and phosphatidyl ethanolamine. Lesser amounts of lysophosphatidyl ethanolamine, cardiolipin, a glycolipid, and two other phospholipids, tentatively identified as lysocardiolipin and lysophosphatidyl glycerol, are also present. Phosphatidyl glycerol content markedly increased, whereas that of phosphatidyl ethanolamine decreased, with increasing salt concentration (0.5 to 3 M NaCl) in the growth medium. These changes result in an increase in the mole ratio of negatively charged polar lipids to neutral polar lipids. Little effect of growth phase on the proportions of the lipid components or their fatty acids was observed in cells grown in 1 M salt. The major fatty acids found were 16:0, 16:1, and 18:1; minor amounts of 14:0, 17:1, and 18:0, and traces of 12:0, 12:1, 14:1, 15:0, 15:1, and 17:0 were also found. The proportions of certain fatty acids also changed with changing salt concentration in the medium.