The seasonal phenology ofZoophthora phytonomi(Arthur) was studied in southern Ontario from 1976 through 1988. The first incidence of disease in the alfalfa weevil,Hypera postica(Gyllenhal), occurred after the accumulation of 204 degree-days (DD) above a base threshold of 9°C from 1 April. In 11 of the 13 yr, epizootics began 57 DD later and lasted for 10 to 14 d, killing up to 99% of the larvae and 50% of the cocooned stages. It is proposed that there are three peaks of conidial production of the fungus in the alfalfa weevil that occur at intervals of 55 to 60 DD. The first occurs when the larvae are small and is coincident with epizootics of the disease in the clover leaf weevil,H. punctata(F.), whose larvae mobilize, increase, and transfer the inocula in early spring. The second occurs at peak incidence of the third alfalfa weevil instar, and the last, just subsequent to peak fourth instar. Larvae infected during the fourth instar die in their cocoons and produce resting spores, which overwinter in the ground litter. The influence of microclimatic factors on epizootics is discussed in relation to threshold densities of the host.