Greenbugs,Schizaphis graminum(Rondani), of biotypes Band E were studied on three wheat genotypes: ‘TAM 105’, susceptible to both biotypes; ‘TAM 107’, “broken resistance” to biotype E but resistant to biotype B; and ‘Largo × TAM 105’, “broken resistance” to biotype B but resistant to biotype E. “Broken resistance” describes a plant genotype resistant to most biotypes but with reduced resistance to one or more biotypes. All combinations of aphid biotype and plant genotype were tested by observing feeding behavior, fecundity and plant damage. Electronic feeding monitors were used for 12 h, after which aphids were reared on the same leaf to obtain data on fecundity and damage. Greenbug biotypes Band E fed less successfully on their respective resistant genotypes than on ‘TAM 105’. The successful or adapted greenbug on a genotype exhibited less baseline and salivation time, a lower number of probes and x-waves, but more phloem ingestion time than the less adapted one. A successful greenbug also produced more offspring and heavier insects, produced less honeydew per one unit of greenbug dry weight gained, and caused a greater reduction in leaf chlorophyll. The broken resistances were variable in all the experiments, but responses were generally intermediate between the susceptible and resistant genotypes, which we interpreted as indicating that they still have some degree of resistance.