Comparisons betweenPsylla pyricolaFoerster populations on a resistant and a susceptible pear topworked into the same trunk stock show that resistance to this pest is due to Ovipositional preference and nymphal antibiosis. Fewer eggs are laid on the resistant cultivar. than on the susceptible cultivar, and nymphal mortality is higher on the resistant cultivar than on the susceptible cultivar. The level of resistance present in the resistant cultivar was sufficient to suppress the pear psylla even though the adjacent susceptible cultivar was defoliated by the pest.