During a study of polysaccharide-hydrolysing bacteria present in different plant sources, two strains were isolated from pulverized decaying wood of Populus alba and classified in the genus Rhizobium on basis of their almost complete 16S rRNA gene sequences. Their closest phylogenetic relatives were Rhizobium galegae USDA 4128(T) and Rhizobium huautlense S02(T), with 98.2 and 98.1 % 16S rRNA gene sequence similarity, respectively. recA and atpD sequence analysis showed that these species have less than 88 and 92 % similarity, respectively, to the novel strains. In contrast to their closest phylogenetic relatives, the two strains showed strong cellulase activity on plates containing CM-cellulose as a carbon source. They were also distinguishable from these species on the basis of other phenotypic characteristics. The strains were able to induce ineffective nodules on Medicago sativa and the sequence of their nodD gene was phylogenetically close to that of Ensifer meliloti 1021 (99.6 % similarity). DNA-DNA hybridization values ranged from 10 to 22 % with respect to R. galegae USDA 4128(T) and 14 to 25 % with respect to R. huautlense S02(T), showing that the strains from this study belong to a novel species, for which the name Rhizobium cellulosilyticum sp. nov. is proposed. The type strain is ALA10B2(T) (=LMG 23642(T)=DSM 18291(T)=CECT 7176(T)).