Abstract
Two strains of the pupal ectoparasite Nasonia vitripennis (Walk.) originally derived from a single laboratory strain exhibited differences in the proportion of adjacent hosts attacked by the parasite following 65 weeks (approximately 32 generations) in experimental parasite–host systems. Strain I parasitized a greater number of adjacent hosts (19.9%) than strain II in the first generation after removal from the multi-cell system. The difference between the two strains and the laboratory strain decreased from the first to the fourth generation indicating a reversion toward a behavior pattern more akin to that of the laboratory strain.