Seedlings of four wheat cultivars were infested with virulent and avirulent Hessian fly,Mayetiola destructor(Say), biotypes. The presence of a single virulent larva resulted in an alteration of resistance in the plant, allowing survival of all normally avirulent larvae that concurrently or subsequently infested the plant. This phenomenon makes possible the genetic loading of a fly population with potentially large numbers of avirulence genes and could, in part, account for the durability of resistance genes following the release of flyresistant wheat germ plasm in agricultural systems. Previous infestation with avirulent larvae did not affect the plants' susceptibility to subsequent infestation by virulent biotypes.