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A novel obligately anaerobic, proteolytic bacterium, designated AP15(T), was isolated from lake sediments of Schirmacher Oasis, Antarctica. The bacterium produced maximum cell mass between 5 and 10 degrees C in an anaerobic basal medium containing 0-5 % tryptone and peptone. The strain grew optimally at a pH around 8(.)0 and tolerated NaCl up to a concentration of 7.5 %. It contained diphosphaticlylglycerol as the major phospholipid and C-15:0,C-16:0 and C-17:0 as the major cellular fatty acids. Several amino acids, including arginine, leucine, isoleucine, cysteine, glutamate and serine, supported growth. Glutamate was degraded to acetate, propionate, CO2 and H-2. In addition, the strain degraded carbohydrates including glucose, raffinose, adonitol, ribose and rhamnose. The main fermentation products during growth on glucose were H2, CO2, formate, acetate, propionate and isovalerate. The DNA G+C content of the bacterium was 24 mol%. On the basis of a phylogenetic analysis, strain AP15T is identified as a close relative of Clostridium subterminale ATCC 25774(T), with which it shares 99-5% similarity at the 16S rRNA gene sequence level; however, it exhibits a low DNA-DNA binding value (55%) to this strain at the whole-genome level. In addition to showing other major differences with respect to C. subterminale and other members of the genus Clostridium, AP15(T) also exhibits phenotypic differences. On the basis of these differences, strain AP15(T) is identified as representing a novel species of the genus Clostridium, for which the name Clostridium schirmacherense sp. nov. is proposed. The type strain is AP15(T) (=DSIM 17394(T)=jCM 13289(T)).