Legume specificity is encoded in rhizobial genetic elements that may be transferred among species and genera. Dissemination (by lateral transfer) of gene assemblies dictating host range accounts for the existence of the same biological variant (biovar) in distinct microbiological species. Different alternative biovars may exist in a single species expanding their adaptation to different niches (legume nodules). A review of all reported biovars is presented. Instead of the term biovar, symbiotic variant (symbiovar) is proposed as a parallel term to pathovar in pathogenic bacteria. Symbiovars should be determined based on the symbiotic capabilities in plant hosts, distinguished by the differences in host range and supported by symbiotic gene sequence information. © 2011 Elsevier GmbH.