It is hypothesized that the interaction between aphids and plants follows a gene-for-gene model. The recent appearance of several new Russian wheat aphid,Diuraphis noxia (Kurdjumov) (Homoptera: Aphididae), biotypes in the United States and the differential response of wheat,Triticum aestivum L., genotypes containing different resistance genes also suggest a gene-for-gene interaction. However, aphid elicitors remain unknown. This study was conducted to identify fractionated Russian wheat aphid extracts capable of eliciting differential responses between resistant and susceptible wheat genotypes. We extracted whole soluble compounds and separated proteins and metabolites from two Russian wheat aphid biotypes (1 and 2), injected these extracts into seedlings of susceptible wheat Gamtoos (dn7) and resistant 94M370 (Dn7), and determined phenotypic and biochemical plant responses. Injections of whole extract or protein extract from both biotypes induced the typical susceptible symptom, leaf rolling, in the susceptible cultivar, but not in the resistant cultivar. Furthermore, multiple injections with protein extract from biotype 2 induced the development of chlorosis, head trapping, and stunting in susceptible wheat. Injection with metabolite, buffer, or chitin, did not produce any susceptible symptoms in either genotype. The protein extract from the two biotypes also induced significantly higher activities of three defense-response enzymes (catalase, peroxidase, and β-glucanase) in 94M370 than in Gamtoos. These results indicate that a protein elicitor from the Russian wheat aphid is recognized by a plant receptor, and the recognition is mediated by theDn7-gene product. The increased activities of defense-response enzymes in resistant plants after injection with the protein fraction suggest that defense response genes are induced after recognition of aphid elicitors by the plant.