Although the aphid Rhopalosiphum padi does not regularly transmit the MAV isolate of barley yellow dwarf virus from singly infected oats, it often transmits MAV, together with the serologically unrelated RPV isolate, from plants doubly infected by MAV and RPV. Vector specificity of the virus isolates appears to be a function of the virus capsid. Some MAV nucleic acid becomes coated with RPV capsid protein during simultaneous synthesis of the two isolates in the doubly infected plant.