We have examined the polar lipids of Clostridium psychrophilum, a recently characterized psychrophilic Clostridium isolated from an Antarctic microbial mat. Lipids were extracted from cells grown near the optimal growth temperature (+5°C) and at -5°C, and analyzed by two-dimensional thin layer chromatography and liquid chromatography coupled with mass spectrometry. The major phospholipids of this species are: cardiolipin, phosphatidylethanolamine, and phosphatidylglycerol. Phosphatidylserine and lyso-phosphatidylethanolamine were found as minor components. The most abundant glycolipids are a monoglycosyldiradylglycerol (MGDRG) and a diglycosyldiradylglycerol (DGDRG). The latter was only seen in cells grown at -5°C. An ethanolamine-phosphate derivative of N-acetylglucosaminyldiradylglycerol was seen in cells grown at -5°C and an ethanolamine-phosphate derivative of MGDRG was found in cells grown at +5°C. All lipids were present in both the all acyl and plasmalogen (alk-1'-enyl acyl) forms with the exception of PS and MGDRG, which were predominantly in the diacyl form. The significance of lipid changes at the two growth temperatures is discussed.