A group of sea-ice-derived psychrophilic bacterial strains possessing the unusual ability to synthesize the polyunsaturated fatty acids eicosapentaenoic acid (20:5 omega 3) and arachidonic acid (20:4 omega 6) belong to the Family Flavobacteriaceae (Flexibacter-Bacteroides-Flavobacterium phylum), according to 16S rRNA sequence analysis. Surprisingly, the isolates were also found to cluster closely to the moderately halophilic and psychrotrophic species [Flavobacterium] gondwanense (sequence similarity 97.8-98.1%). The whole-cell fatty acid profiles of this group and [Flavobacterium] gondwanense were very similar and distinct from other related flavobacteria. The sea ice strains and [Flavobacterium] gondwanense differed substantially in terms of ecophysiology, possibly representing divergent adaptations to sympagic and planktonic marine habitats, respectively. Evidence based on phylogeny and fatty acid profiles supports the conclusion that the taxa are close relatives distinct from other bacterial groups. It is thus proposed that the sea ice strains represent a novel taxon designated Psychroflexus torquis gen. nov., sp. nov. (type strain ACAM 623T) while [Flavobacterium] gondwanense becomes Psychroflexus gondwanense gen. nov., comb. nov.