The duration of feeding and relative amounts consumed by Hessian fly,Mayetiola destructor(Say), larvae on resistant and susceptible seedling wheat plants labeled with [35S]L-methionine was investigated. Plants were labeled over 48-h periods from initial infestation to 8 d after infestation, and the uptake of35S by larvae determined. With the H3and H5genes for resistance, the uptake of35S by larvae was significantly less than on susceptible controls, and suggested that the larvae were unable to initiate or maintain sustained feeding on the resistant plants. With the H6gene for resistance, uptake of35S by larvae during the first 48 h was not significantly different from that on susceptible controls, but then declined between 48 and 96 h. These data indicate that with the H6gene, larvae fed actively for the first 48 h after infesting the plants, then ceased their feeding between 48 and 96 h. All larvae on resistant plants were alive 48 h after infesting plants and approximately 60% were alive 96 h after infesting resistant plants. All larvae on resistant plants were dead by 144 h after infestation. Results are discussed in the context of possible mechanisms of action for the resistance genes investigated.