The individual polar lipids produced by Psuedomonas vesicularis NCTC 10 900 during surface culture have been isolated. The major lipids are phosphatidylglycerol, a phosphatidyl-alpha-D-glucopyranosyldiacylglycerol, 1,2-di-O-acyl-3-O-alpha-D-glucopyranosylglycerol. 1,2-di-O-acyl-3-O-alpha-D-glucopyranuronosylglycerol, and 1,2-di-O-acyl-3-O[beta-D-glucopyranosyl-(1 yields 4)-alpha-D-glucopyranuronosyl]glycerol. These are also the major polar lipids of Pseudomonas diminuta. Additional lipids present in P. vesicularis are unidentified carotenoids and a novel lipid characterised as 1,2-di-O-acyl-3-O-alpha-D-glycero-D-glucoheptopyranosylglycerol. A cis-octadecenoic acid and hexadecanoic acid are the major fatty acids: C15 and C17 acids are significant minor components. The fatty methyl ester fractions derived from three of the lipids (most notably the glucosyldiacylglycerol) contained substantial amounts of a compound with chromatographic properties resembling those of an octadecenoic ester: the identity and origin or this compound remained uncertain.