AbstractIn tests with 4 RRSV naturally occurring strs., virus hybrids were made by mixing RNA-1 and RNA-2 preparations from different strs. Parent strs. were regenerated by crossing appropriate hybrids. In the crosses both serological specificity and transmissibility by Longidorus elongatus were determined by RNA-2, suggesting that the protein surface of the virus particles is involved in the transmission process. Ability of isolates to cause systemic yellowing in Petunia hybrida, previously found to be also controlled by RNA-2, was shown to be associated with distinctive ultrastructural changes in the chloroplasts. Severity of systemic symptoms in Chenopodium quinoa and other herbaceous hosts, ability to infect Lloyd George raspberry and to invade the non-inoculated leaves of Phaseolus vulgaris, were all determined by RNA-1. Both RNA species played a part in determining lesion type in inoculated leaves of C. quinoa and C. amaranticolor; in some crosses the 2 kinds of hybrid were respectively less virulent and more virulent than either parent. The determinant for systemic symptoms in P. hybrida that is carried by RNA-2 was not expressed when in association with RNA-1 from the str. able to infect Lloyd George raspberry. Some genes of RRSV are probably pleiotropic.