The inheritance of resistance in oats,Avena sativaL. andA. byzantinaK. Koch, to greenbug,Schizaphis graminum(Rondani), biotypes B and C was studied. Results from F2progeny indicated that PI 186270 and CI 1580 had different genes or different loci which control resistance to biotype C. Each had a single completely dominant gene pair. CI 4888 had a separate source of resistance for biotype B, and resistance was also probably conditioned by a single major gene with complete dominance. The gene that controlled resistance to one biotype was not effective against the other biotype. All genes appeared to be independently inherited.