Members of the geminvirus group of plant viruses collectively infect a broad spectrum of species. Individual viruses which are genetically very similar may nevertheless have distinct host ranges. Two such geminiviruses are tomato golden mosaic virus (TGMV) and been golden mosaic virus (BGMV), for which common hosts have not previously been reported. Each virus has two genome components, designated A and B. The A component is capable of autonomous replication and encapsidation, whereas the B component provides viral functions required for the spread of infection in plants. To investigate the basis for the distinctive host ranges of BGMV and TGMV, we have introduced plasmids containing cloned viral genome components into Nicotiana benthamiana, a good host for TGMV, and bean, Phaseolus vulgaris, a good host for BGMV. We found that TGMV has a low specific infectivity for bean and is virulent, whereas BGMV has a high specific infectivity for N. benthamiana, but infections are asymptomatic and viral DNA accumulation is low. To investigate which viral functions were defective in the poor host in each case, we attempted to complement them by co-inoculation with the well-adapted virus. After inoculation of beans with both viruses, only BGMV was detected. Thus, TGMV exhibits a noncomplementable host adaptation defect in beans. This suggests that the defect has a cis-acting or virus-specific trans-acting genetic basis. In contrast, the BGMV phenotype of low DNA accumulation in N. benthamiana was partially complemented by TGMV A alone and complemented further by the complete TGMV genome. This suggests that a virus nonspecific, trans-acting factor encoded by the BGMV A component is poorly adapted to N. benthamiana. The results of this study indicate that bipartite geminivirus host range may be limited by defective virus-host interactions of more than one kind.