In Nicotiana benthamiana, the common strain of the bipartite geminivirus Tomato golden mosaic virus (csTGMV) induces extensive chlorosis whereas the yellow vein strain (yvTGMV) produces veinal chlorosis on systemically infected leaves. In Datura stramonium, csTGMV produces leaf distortion and a severe chlorotic mosaic whereas yvTGMV produces only small chlorotic lesions on systemically infected leaves. Genetic recombination and site-directed mutagenesis studies using infectious clones of csTGMV and yvTGMV have identified a role in symptom production for the gene encoding the movement protein (MP). The MP amino acid at position 272, either valine (csTGMV) or isoleucine (yvTGMV), influenced symptoms in both hosts by inducing an intermediate phenotype when exchanged between the two strains. Exchange of an additional strain-specific MP amino acid at position 288, either glutamine (csTGMV) or lysine (yvTGMV), resulted in the change of symptom phenotype to that of the other strain. In situ hybridization analysis in N. benthamiana demonstrated that there was no qualitative difference in the tissue distribution of the two strains although csTGMV accumulated in higher amounts, suggesting that the efficiency of virus movement rather than distinct differences in tissue specificity of the strains is responsible for the symptom phenotypes.