A novel filamentous bacterium, strain SCSIO 10219(T), was isolated from a sediment sample collected from the South China Sea (113° 3.752' E 18° 1.722' N) at a depth of 2105 m. Growth was observed at 25-35 °C (optimum 30 °C) and pH 5.0-8.0 (optimum pH 6.0-7.0). The organism formed yellow-white colonies with radial wrinkles. Aerial mycelium was not produced on any of the growth media tested. Phenotypic characterization and 16S rRNA gene sequence analysis indicated that strain SCSIO 10219(T) belongs to the family Thermoactinomycetaceae. The strain contained ll-diaminopimelic acid in the cell wall. The predominant menaquinone was MK-7. The phospholipids were diphosphatidylglycerol, phosphatidylglycerol, phosphatidylmethylethanolamine, phosphatidylethanolamine and five unknown phospholipids. Major fatty acids were anteiso-C(15:0) and iso-C(15:0). The DNA G+C content was 46.5 mol%. On the basis of chemotaxonomic properties and phylogenetic analysis based on 16S rRNA gene sequence data, it is proposed that this strain represents a novel species in a new genus, Marininema mesophilum gen. nov., sp. nov., in the family Thermoactinomycetaceae. The type strain of the type species is SCSIO 10219(T) ( = CCTCC AA 2011006(T) = DSM 45610(T)). In addition, we propose that the description of the family Thermoactinomycetaceae should be further emended based on the present study.