The analysis of nod genes and 16S rRNA gene regions, Nod factors, and nodulation abilities of Brady rhizobium strains isolated from tropical Thai Vigna species is reported. A total of 55 Bradyrhizobium strains isolated from two cultivated and six wild Vigna species growing in central and northern Thailand were evaluated. Thai Vigna spp. Bradyrhizobium strains showed higher levels of nod gene RFLP diversity compared with Thai soybean Brady rhizobium strains or temperate strains of Bradyrhizobium japonicum and Bradyrhizobium elkanii. Analysis of the 16S rRNA gene region using selected strains also suggests a high genetic diversity of the Thai Vigna-Bradyrhizobium association. Based on thin-layer chromatography analysis, Nod factors produced by tropical Thai Vigna spp. Brady rhizobium strains are more diverse than temperate Japanese and US strains of B. japonicum and B. elkanii. Thai Vigna spp. Bradyrhizobium strains showed variation in nodulation ability and affinity, estimated by the number of normal nodules versus green nodules in an inoculation study. There are some Bradyrhizobium-host combinations that could not form any nodules, suggesting that some genetic differentiation has evolved in their host range. However, most of the Thai Vigna spp. Bradyrhizobium strains formed nodules on the cultigens soybean (Glycine max), mungbean (Vigna radiata), azuki bean (Vigna angularis), and cowpea (Vigna unguiculata). This is the first study on Bradyrhizobium strains associated with a range of cultivated and wild Vigna and reveals that these Bradyrhizobium strains are diverse and may provide novel sources of useful variation for the improvement of symbiotic systems.