RBDV-RB readily infects by grafting many raspberry cultivars that have gene Bu, which confers strong resistance or immunity to isolates of the common str. Haida is one of 2 cultivars that are highly resistant to or immune from RBDV-RB, but both its parents, Creston and Malling Promise, are infectible. Studies of the segregation of resistance to both RBDV-RB and a common str. of RBDV (D200) in 4 progenies related to Haida or its parents, showed that resistance to RBDV-RB was heritable and occurred when gene Bu was present with a second resistance component whose inheritance is probably multigenic. It was thought that the second component might be a form of partial resistance to graft inoculation of varying expression, and that Haida possesses this resistance at a high level that has not been distinguished from immunity in the graft inoculations used. Cultivars Creston and Malling Promise possibly have this resistance to a lesser degree, while resistance in Heritage has been distinguished from immunity only by extensive graft tests. Some possible implications for breeding RBDV-RB resistant cultivars are discussed.