One hundred and seventy three bacterial strains, isolated previously after enrichment under oligotrophic, psychrophilic conditions from Arctic (98 strains) and Antarctic seawater (75 strains), were characterized by gas-liquid chromatographic analysis of their fatty acid compositions. By numerical analysis, 8 clusters, containing 2 to 59 strains, could be delineated, and 8 strains formed separate branches. Five clusters contained strains from both poles, two minor clusters were confined to Arctic isolates, and one cluster consisted of Antarctic isolates only. The 16S rRNA genes from 23 strains, representing the different fatty acid profile clusters and including the unclustered strains, were sequenced. The sequences grouped with the alpha and gamma Proteobacteria, the high percent G+C Gram-positives, and the Cytophaga-Flavobacterium-Bacteroides branch. The sequences of strains from 4 clusters and of 7 unclustered strains were closely related (sequence similarities above 97%) to reference sequences of Sulfitobacter mediterraneus, Halomonas variabilis, Alteromonas macleodii, Pseudoalteromonas species, Shewanella frigidimarina, and Rhodococcus fascians. Strains from the other four clusters and an unclustered strain showed sequence similarities below 97% with nearest named neighbours, including Rhizobium, Glaciecola, Pseudomonas, Alteromonas macleodii and Cytophaga marinoflava, indicating that the clusters which they represent form as yet unnamed taxa.