A Gram positive, thermophilic, and filamentous bacterium, designated strain GIESS004(T), was isolated from a sludge compost from Guangzhou, China. Growth occurred at 45-55 A degrees C (optimum 50-53 A degrees C) and pH 6.0-10.0 (optimum pH 7.0-7.5). The organism formed cream-yellow colonies with radial wrinkles. Aerial mycelium was not produced in any of the growth media tested. Single spores were produced along the substrate hyphae. Strain GIESS004(T) contained MK-7 as the predominant menaquinone and iso-C-16:0, iso-C-15:0 and iso-C-17:0 as the major cellular fatty acids. The cell wall contained meso-diaminopimelic acid. The DNA G+C content was 60.7 mol%. Phylogenetic analyses based on 16S rRNA gene sequences indicated that strain GIESS004(T) was associated with the genus Planifilum and formed a cluster with its closest relative of Planifilum composti P8(T) (99.5 % similarity). However, the DNA-DNA pairing study showed that the isolate displayed reassociation value of only 57.6 +/- A 2.3 % with P. composti P8(T). On the basis of the polyphasic taxonomic results, strain GIESS004(T) (= CCTCC AB2013107(T) = MCCC 1K00414(T)) is designated as the type strain of a novel species of the genus Planifilum, for which the name Planifilum caeni sp. nov. is proposed.