Nodule isolates from 11 species of wild legumes in north-western China were characterized by numerical taxonomy, PCR-based 165 rRNA gene RFLP and sequence analyses, DNA-DNA hybridization, restriction patterns of nodDA5 and nifH genes, and symbiotic propetties. Based on the results of numerical taxonomy, most of the 35 new isolates were grouped into five clusters (clusters 7, 9, 12, 14 and 15). Clusters 7 and 12 were identified as Mesorhizobium amorphae and Agrobacterium tumefaciens, respectively, based on their high DNA homologies with the reference strains for these species, their 165 rRNA gene analysis and their phenotypic features. Results of 16s rDNA PCR-RFLP analysis showed that cluster 9 belonged to Rhizobium. Clusters 14 and 15 were identified as Mesorhizobium based on their moderately slowgrowing, acid-producing characters and the high similarity of their 165 rDNA PCR-RFLP patterns to those of Mesorhizobium species. These two clusters were genomic species distinct from all described species based on analysis of DNA relatedness within this genus. The isolates in cluster 12 (Agrobacterium tumefaciens) failed to nodulate their original host and other selected hosts and they did not hybridize to nif or nod gene probes. The possibility of opportunistic nodulation of these isolates is discussed. Identical restriction patterns were obtained in the nif or nod gene hybridization studies from the three isolates within cluster 15, which were isolated from the same host species. The isolates from different host plants in each of clusters 9 and 14 produced different nodDA5 RFLP patterns, but similar nifH RFLP patterns appeared (one band for each isolate). Different patterns were observed among different clusters from both the nod and nif gene hybridization studies. Crossnodulation was recorded among the isolates and the host plants in the same cluster and promiscuous properties were found among some of the hosts tested.