Population densities of larvae of the alfalfa weevil,Hypera postica(Gyllenhal), and prevalence of parasitism byBathyplectes curculionis(Thomson) were determined in Grady and Payne Counties of Oklahoma from 1976 to 1987. After initial detection of the fungal entomopathogen,Erynia phytonomi(Arthur), in 1983, additional sampling was done to estimate the extent of fungus-induced mortality in alfalfa weevil populations. In addition, weevil larvae were dissected to determine the proportion of those infected by the fungus that also contained eggs or larvae ofB. curculionis. Epizootics of the fungus at both research sites coincided with the highest prevalence of parasitism by the second generation ofB. curculionisin 1983 through 1986, and reduced survival of the parasitic wasp by>90%. Although competition between the pathogen and parasite for hosts has resulted in a high level of mortality ofB. curculionison an annual basis, long-term reductions in parasite populations are yet to be confirmed.