The eight dominant culturable members of an Antarctic fellfield soil bacterial community were four Arthrobacter species, Sanguibacter suarezii, Aureobacterium testaceum, a Bacillus sp., and a Pseudomonas sp.. All of the isolates grew at 2°C, but two of the Arthrobacter spp. were psychrophilic, while the other six bacterial species were psychrotolerant. However, the fastest growing organisms at low temperatures were not the psychrophiles, and the psychrotolerant Bacillus sp. grew fastest at temperatures up to 25°C. When the growth temperature of cultures was altered, the phospholipid content of the two psychrophilic Arthrobacter spp. decreased, whereas the phospholipid contents of the psychrotolerant spp. either increased or did not change. Only one psychrophilic and one psychrotolerant Arthrobacter sp. modified its polar lipid head-group composition in response to a lowering of growth temperature. The change in Arthrobacter sp. CL2-1 was particularly marked and novel in that at low temperatures phosphatidylethanolamine was replaced completely by a phosphoglycolipid and phosphatidylserine, neither of which was present at higher growth temperatures. All eight isolates altered the fatty acyl compositions of their membrane lipids in a manner that was only partially dependent on taxonomic status. In Bacillus sp. C2-1 the changes were opposite to that predicted on the basis of membrane fluidity considerations. The isolates used different combinations of changes in fatty acid branching, unsaturation and chain length. There was no single strategy of thermal adaptation that was employed and the variety of strategies used did not follow phylogenetic boundaries.