Field populations of overwintered alfalfa weevil adults were studied in central New York from 1971–73 to measure the impact of the parasiteMicroctonus aethiopoideson ovipositIOn of selected alfalfa weevil populations. Seasonal peak parasitism of overwintered weevils ranged from 20–88% and oviposition by parasitized alfalfa weevil populations was reduced an avg of 47%. Host oviposition reduction was due to sterilization of female weevils and their subsequent premature death as compared to unparasitized weevils. The timing and magnitude of parasite oviposition relative to that of host oviposition was the key factor in determining the impact of this parasite on the oviposition potential of its host.