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Members of the genus Agrobacterium constitute a diverse group of organisms, all of which, when harbouring the appropriate plasmids, are capable of causing neoplastic growths on susceptible host plants. The agrobacteria, which are members of the family Rhizobiaceae, can be differentiated into at least three biovars, corresponding to species divisions based on differential biochemical and physiological tests. Recently, Young et al. [Int J Syst Evol Microbiol 51 (2003), 89–103] proposed to incorporate all members of the genus Agrobacterium into the genus Rhizobium. We present evidence from classical and molecular comparisons that supports the conclusion that the biovar 1 and biovar 3 agrobacteria are sufficiently different from members of the genus Rhizobium to warrant retention of the genus Agrobacterium. The biovar 2 agrobacteria cluster more closely to the genus Rhizobium, but some studies suggest that …