The relationship between the parasite,Microctonus aethiopoides, and its host, the alfalfa weevil, was studied in Ontario during 1974 and 1975. In the field, the populations of the 1st generation adults of the parasite were synchronized well with those of the overwintered weevils, resulting in parasitism of 76–82%. The 2nd generation of the adult parasites and the summer adult weevils overlapped for about 2 wk and parasitism of the diapausing host was 41–60%. Some adults ofM. aethiopoideswere present in the field before the summer adults emerged and the significance of this with respect to the establishment and carry over of the parasite from one year to the next is discussed. Parasitism byM. aethiopoidesdid not significantly affect the mortality of the diapausing alfalfa weevil either in the field or during low temperature storage in growth cabinets.