An unusual sulphate-reducing bacterium was isolated from Lake Fryxell, Antarctica. Designated strain FESu, it illustrates the difficulty in assigning some isolates of Desulfovibrio spp. to a described species. FESu was characterized by its relatively low mol% G + C (45%), its rod-shaped morphology, inability to grow on lactate, pyruvate, malate or choline in the absence of sulphate, and by its lack of desulfoviridin. Characterization of isolatesElectron microscopy, gas chromatography, and determination of the mol% G + C by its melting temperature were all as described by Patel et al.