A bacterial strain designated XIL08(T) was isolated from an elm tree affected by Dutch elm disease. Strain XIL08(T) is Gram-positive, aerobic, rod-shaped and non-motile. The complete 16S rDNA sequence of this micro-organism was obtained and phylogenetic analysis based on the neighbour-joining method indicated that the closest related organism belongs to the genus Xylanimonas of the family Promicromonosporaceae, suborder Micrococcineae. Cell-wall analyses revealed the presence of type A4alpha, L-lys-L-ala-D-Glu peptidoglycan. The cell-wall sugars found were rhamnose in large amounts, fucose, mannose and galactose and traces of arabinose and glucose. HPLC analysis of menaquinones revealed two peaks, the main peak corresponding to MK-9(H(4)) and the smaller one to MK-8(H(4)). The major fatty acid found was anteiso-C(15 : 0). Mycolic acids were absent. The polar lipids detected were phosphatidylglycerol, diphosphatidylglycerol, phosphatidylinositol and phosphatidylinositol mannosides. The G+C content of the DNA was 72 mol%. Isolate XIL08(T) hydrolysed xylan but not cellulose. Growth was observed with many carbohydrates including acetate and xylan as the only carbon source. Catalase activity was not detected. The data from this polyphasic study suggest that this bacterium belongs to a novel genus of the family Promicromonosporaceae. It is proposed that isolate XIL08(T) (=LMG 21721(T)=CECT 5731(T)) be classified in a new genus, Xylanibacterium gen. nov., as the type strain of Xylanibacterium ulmi sp. nov.