The mean weight of seed per plant and the mean plant weight of oats inoculated jointly with a Rhopalosiphum padi-specific isolate and a Macrosiphum avenae-specific isolate of barley yellow dwarf virus or with the R. padi-specific isolate and a non-specific isolate, were significantly less than those of oats singly inoculated with the respective isolates. Values for these criteria for oats jointly inoculated with the M. avenae-specific and the non-specific isolates were intermediate between, or were greater than, those for singly inoculated oats. This was true whether inoculation with the individual isolates was made simultaneously or with an interval of 16 days between the two inoculations.When oats were inoculated jointly with two Schizaphis graminum-specific isolates, seed and plant weights were significantly less than on plants inoculated singly with each isolate.For each combination used, both isolates were transmissible to oats from jointly inoculated plants. It is inferred that interference occurred only between the M. avenae-specific and the non-specific isolates and that a synergistic response resulted with the other combinations.