In a growth chamber test,Lysiphlebus testaceipes(Cresson) was used to parasitize biotypes C and E of the greenbug,Schizaphis graminum(Rondani), on two selections of oats. One oat cultivar, ‘C.I. 4888,’ was resistant to the greenbug, whereas the other, ‘Nora,’ was susceptible to all known greenbug biotypes in the great plains. Even though the plant resistance had no significant effect on the fecundity of the parasitoid, the test proved that the recent greenbug biotype change had not reduced the effectiveness ofL. testaceipes, and that plant resistance and biological control could be compatible components in pest management of the greenbug