A novel thermophilic, strictly anaerobic, chemo-organotrophic bacterium, designated MC(T), was isolated from a geothermally heated sediment of a marine hydrothermal system at Palaeochory Bay, Milos, Greece. Cells of strain MC(T) were rods of variable length (4-12 mum) and width (0.2-0.3 mum), occurring as single cells or forming large aggregates that were visible as flocs. Strain MC(T) grew optimally at pH 7.0 and 60 degrees C and with 3 % (w/v) NaCl. Strain MC(T) grew chemo-organoheterotrophically and fermented peptides and di- and polysaccharides in the presence of 0.1 g yeast extract l(-1). The DNA G+C content of strain MC(T) was 43.3 mol%. Phylogenetic analysis of the 16S rRNA gene sequence placed strain MC(T) within the genus Caldithrix. However, strain MC(T) possessed certain phenotypic features that differentiated it from the type strain of the only species of the genus Caldithrix described to date. On the basis of phylogenetic and phenotypic characteristics, it is proposed that strain MC(T) represents a novel species, Caldithrix palaeochoryensis sp. nov. The type strain is MC(T) (=DSM 21940(T) =VKM B-2536(T)). In addition, an emended description of the genus Caldithrix is presented.