The taxonomic position of five actinobacterial strains isolated from dust, an animal shed, the air inside a museum and soil was investigated using a polyphasic approach. The growth characteristics were unusual for actinomycetes. Optimal growth was at temperatures ranging from 2 to 10 degrees C. After small-step adaptation (5 degrees C steps) to higher temperatures, the strains were also able to grow at 20 degrees C. Cell wall analyses revealed that the organisms showed a hitherto undescribed, new group B-type peptidoglycan [type B2beta according to Schleifer & Kandler (1972), but with lysine instead of ornithine]. All strains contained menaquinone MK-9. Mycolic acids were not detected. Diphosphatidylglycerol, phosphatidylglycerol and an unknown glycolipid were detected in the polar lipid extracts. The main fatty acids were 12-methyl-tetradecanoic acid (15:0 anteiso), 12-methyl-tetradecenoic acid (15:1 anteiso), 14-methyl-pentadecanoic acid (16:0 iso) and 14-methyl-hexadecanoic acid (17:0 iso), as well as an unusual compound identified as 1,1-dimethoxy-anteiso-pentadecane (15:0 anteiso-DMA). The G+C content of DNA was approximately 71 mol%. The results of 16S rRNA gene sequence comparisons revealed that the strains represent a new lineage in the suborder Micrococcineae and the family Microbacteriaceae of the order Actinomycetales. On the basis of these results the new genus Frigoribacterium gen. nov. is proposed, harbouring the new species Frigoribacterium faeni sp. nov. (type strain = 801T = DSM 10309T).