Small numbers of larval wheat midge, Sitodiplosis mosellana Géhin, survived and matured in each of five field seasons in a plot of spring wheat carrying the Sm1 gene for antibiosis resistance against this insect. Wheat midge developing on resistant wheat had higher survival in the laboratory than in the field, but survival was always very low compared with that of larvae developing on susceptible wheat. The mass of these larvae and their survival during diapause were approximately half those of larvae developing on susceptible wheat in both the laboratory and the field. The survival of some wheat midge larvae on resistant wheat, and their reduced mass, is consistent with the hypothesis that a virulence allele allowing adaptation to Sm1 is present in the population. Assuming this to be the case, the frequency of the allele in the population was estimated to be between 0.8 × 10⁻⁴ and 1.6 × 10⁻², if surviving larvae are heterozygous for the allele. Although rare, a virulence allele occurring at this frequency would likely allow the wheat midge to overcome the resistance gene Sm1 once resistant wheat is grown over a wide area.