In this study reasons are suggested why virulent biotypes ofSchizaphis graminum(Rondani) (the greenbug) are severely injurious to winter wheat (Triticum aestivumL.) Biotype C greenbugs (GBC) were compared with biotype E greenbugs (GBE)on a winter wheat selection with ‘Amigo’-type resistance (resistant to GBC, susceptible to GBE). Superior growth and reproduction of GBE was associated with feeding damage (chlorosis and necrosis) and greatly increased concentrations of free amino acids in the wheat leaves at the infestation sites. GBC did not elicit this type of damage during the course of the experiment. When GBC fed on tissue previously damaged by GBE, it reproduced and grew as if feeding on a susceptible host. The nitrogen concentration of these aphids and their honeydew was also greater than that of GBC fed from GBC-damaged tissue. Thus, ‘Amigo’-type resistance was apparently not solely dependent upon secondary metabolites or other substances that may render the plant unpalatable or toxic. Rather, it appeared to be related to a chemical interaction between the susceptible host and the greenbug which resulted in a localized modification of the plant's metabolism similar to senescence. By inducing chlorosis, virulent greenbugs appear to enhance the nutritional quality of their hosts.