Biotype C of the greenbug,Schizaphis graminum(Rondani), was cultured in the greenhouse on resistant sorghum entries for ca. 2 years. About every 4 mo the components of resistance of the plant were measured to determine whether new biotypes had occurred. Although non-preference and tolerance did not change significantly within an entry, the fecundity of aphids on the resistant entries decreased. This reduction persisted when the greenbugs were transferred directly to susceptible “Wheatland” and then tested, but it did not persist when they were left on Wheatland 4 mo before testing. Thus the reduced reproduction was probably a conditioning effect and not a mutation.