A novel filamentous bacterium, designated strain JIR-001(T), was isolated from hemipelagic sediment in deep seawater. This strain was non-motile, Gram-positive, aerobic, heterotrophic and thermophilic; colonies were of infinite form and ivory coloured with wrinkles between the centre and the edge of the colony on ISP2 medium. The isolate grew aerobically at 55-73 °C with the formation of aerial mycelia; spores were produced singly along the aerial mycelium. These morphological features show some similarities to those of the type strains of some species belonging to the family Thermoactinomycetaceae. Phylogenetic analysis based on 16S rRNA gene sequences confirmed that strain JIR-001(T) belongs to the family Thermoactinomycetaceae within the class Bacilli. Similarity levels between the 16S rRNA gene sequence of strain JIR-001(T) and those of the type strains of Thermoactinomycetaceae species were 85.5-93.5%; highest sequence similarity was with Melghirimyces algeriensis NariEX(T). In the DNA-DNA hybridization assays between strain JIR-001(T) and its phylogenetic neighbours the mean hybridization levels with Melghirimyces algeriensis NariEX(T), Planifilum fimeticola H0165(T), Planifilum fulgidum 500275(T) and Planifilum yunnanense LA5(T) were 5.3-7.5, 2.3-4.7, 2.1-4.8 and 2.5-4.9%, respectively. The DNA G+C content of strain JIR-001(T) was 55.1 mol%. The major fatty acids were iso-C15:0, iso-C17:0, iso-C16:0 and C16:0. The polar lipid profile consisted of diphosphatidylglycerol, phosphatidylethanolamine, phosphatidylmonomethylethanolamine, phosphatidylglycerol, glucolipid, phosphatidylserine, an amino-group containing phospholipid, an unknown phospholipid and two unknown lipids. The predominant menaquinone was MK-7 and the cell-wall peptidoglycan contained meso-diaminopimelic acid, glutamic acid and alanine. On the basis of phenotypic characteristics and 16S rRNA gene sequence comparisons, strain JIR-001(T) is considered to represent a novel species in a new genus of the family Thermoactinomycetaceae, for which the name Polycladomyces abyssicola gen. nov., sp. nov. is proposed. The type strain of Polycladomyces abyssicola is JIR-001(T) (=JCM 18147(T)=CECT 8074(T)).