Infection rates ofZoophthora phytonomi(Arthur) Batko in alfalfa weevil,Hypera postica(Gyllenhal), larvae were monitored from 1979 through 1981 in Rockbridge County, Va. Infection rates of the pathogen were highest near the end of the spring alfalfa crop when alfalfa weevil larval populations were declining.Z. phylonomiwas effective in suppressing alfalfa weevil populations below economic threshold levels only during a period of higher rainfall that occurred in 1979. A negative correlation between infection rates ofZ. phytonomiand parasitization rates of the ichneumonidparasiteBathyplectes anurus(Thomson) (Hymenoptera: Ichneumonidae) suggests a possible interference between these two natural control agents.