The taxonomic status of an actinomycete (MA 140/96T) isolated from indoor building materials of a children's day care centre was studied using the polyphasic approach. The cell morphology was atypical for an actinomycete, electron microscopy revealed a hairy surface, highly unusual for Gram-positive bacteria. The organisms grew at 10-37 degrees C, no growth was visible at 5 degrees C and 45 degrees C in 5 d. The cell wall contained the diamino acid meso-diaminopimelic acid and the sugars arabinose, galactose, mannose and ribose. The phospholipids phosphatidylethanolamine, phosphatidylinositol, phosphatidylglycerol and diphosphatidylglycerol were detected. The only menaquinone found was MK-9(H2). The fatty acid pattern was composed of palmitic acid (23.6%) palmitoleic acid (16.5%) and another hexadecenoic acid 16:1cis11 (1.4%), oleic acid (29.9%), stearic acid (2.9%) and the 10-methyl-branched tuberculostearic acid (23.3%). A gas-chromatographic analysis of the mycolic acid revealed a carbon-chain length of C50-C56. The G + C was 64.8 mol%. The results of 16S rDNA sequence comparisons revealed that strain MA 140/96T represents a new lineage in the suborder Corynebacterineae of the order Actinomycetales. Therefore, it was concluded that strain MA 140/96T should be assigned to a new genus and species, for which the name Williamsia muralis gen. nov., sp. nov. is proposed. The type strain of the species is MA 140/96T (= DSM 44343T).