Since first being discovered in Oklahoma during 1983, Erynia phytonomi (Arthur) has been found in 35 counties. Seasonal incidence of the pathogen and the percentage of mortality in populations of the alfalfa weevil, Hypera postica (Gyllenhal), were determined at two locations in Oklahoma from 1983 to 1987. The first infections were usually detected in weevils after accumulation of 200–250 degree-days (base temperature l0°c) from 1 January, coinciding closely with occurrence of peak larval densities. During each year except 1987, rainfall and temperature conditions were favorable to the pathogen, and disease incidence in host larvae exceeded 60% on one or more sampling dates. Incidence of infection in prepupae frequently exceeded 80% and up to 52% of pupae were infected. The pathogen appears to be important as a natural control agent and often reduces the number of insecticide applications required.