Lipid preparations from the cells of a moderately halophilic bacterium, Pseudomonas halosaccharolytica grown under the two extreme conditions of high temperature-high NaCl concentration and low temperature-low NaCl concentration showed distinctively different profiles in phospholipid and fatty acid composition. Cells grown at 40°C in medium containing 3.5M NaCl had high concentrations of saturated and C₁₉ cyclopropanoic fatty acids (about 50 per cent of the total), whereas cells grown at 20°C in medium containing 0.5M NaCl had decreased concentrations of these fatty acids with increased concentrations of the corresponding unsaturated fatty acids. The phospholipid composition was also affected by the culture conditions; cells grown at 40°C in 3.5M NaCl had large amounts of acidic phospholipids, whereas those grown at 20°C in 0.5M NaCl had small amounts. ESR studies on liposomes prepared from lipids of cells grown under the two conditions showed characteristic profiles for correlation times and order parameters of three spin labels of stearic acid derivatives similar to those of membranes of whole cells of this bacterium. ESR studies showed that the physical properties of the liposomes from the total extractable lipids and isolated phosphatidylglycerol from the cells were completely different from those of synthetic dioleoyl-phosphatidylglycerol. Liposomes of the lipids extracted from cells grown at 40°C in 3.5M NaCl showed change in rotational viscosity on altering the NaCl concentration to 0.5M, whereas liposomes of lipids extracted from cells grown at 20°C in 0.5M NaCl did not show change in rotational viscosity on increasing the NaCl concentration to 3.5M.