In selective host species, the extent of Citrus tristeza virus (CTV) infection is limited through the prevention of long-distance movement. As CTV infections often contain a population of multiple strains, we investigated whether the members of a population were capable of interaction, and what effect this would have on the infection process. We found that the tissue-tropism limitations of strain T36 in selective hosts could be overcome through interaction with a second strain, VT, increasing titer of, and number of cells infected by, T36. This interaction was strain-specific: other strains, T30 and T68, did not complement T36, indicating a requirement for compatibility between gene-products of the strains involved. This interaction was also host-specific, suggesting a second requirement of compatibility between the provided gene-product and host. These findings provide insight into the 'rules' that govern interaction between strains, and suggest an important mechanism by which viruses survive in a changing environment.