A bacterial strain, N2-680(T) (=DSM 15264(T)=LMG 21760(T)), isolated from a thermophilic aerobic digester of municipal sludge, was characterized with respect to its morphology, physiology and taxonomy. Phenotypically, the isolate was a Gram-negative rod with a polar flagellum, catalase- and oxidase-positive, containing cytoplasmic inclusions of poly-beta-hydroxybutyrate and had an optimal growth temperature of about 47 degrees C. Strain N2-680(T) was unable to reduce nitrate and could use organic acids, amino acids and carbohydrates as single carbon sources. Chemotaxonomic analysis revealed that ubiquinone 8 was the major respiratory quinone of this organism and that phosphatidylethanolamine and phosphatidylglycerol were the major polar lipids. At 50 degrees C, the major components in fatty acid methyl ester analysis were C(16 : 0) and cyclo-C(17 : 0). The highest 16S rDNA sequence identity of isolate N2-680(T) was to Leptothrix mobilis and Ideonella dechloratans (95.7 %) and to Rubrivivax gelatinosus and Aquabacterium commune (95.6 %). 16S rDNA sequence similarities to species of two related thermophilic genera, Caldimonas manganoxidans and Tepidimonas ignava, were lower (93.6 and 94.7 %). On the basis of phylogenetic analyses and physiological and chemotaxonomic characteristics, it is proposed that isolate N2-680(T) represents a new genus and species, for which the name Caenibacterium thermophilum gen. nov., sp. nov. is proposed.