The lipids of a moderately halophilic methanogen, Methanococcus voltae, accounted for 5.1% of the cell dry weight and consisted of 91% polar lipids and 9% neutral lipids. Twelve polar lipids were detected, three of which, all derivatives of 2,3-di-O-phytanyl-sn-glycerol, were identified as: 2,3-di-O-phytanyl-1-O-[beta-D-glucopyranosyl-(1-6)-beta-D- glucopyranosyl]-sn-glycerol, 2,3-di-O-phytanyl-1-O-[beta-D-glucopyranosyl]-sn-glycerol and a novel NAc-glucosamine 1-phosphate diether, and 2,3-di-O-phytanyl-1-[phosphoryl-2-acetamido-2-deoxy-beta-D- glucopyranosyl]-sn-glycerol. The neutral lipids consisted mainly of squalenes: squalene, dihydrosqualene, tetrahydrosqualene, hexahydrosqualene, and unidentified squalenes.