A dilution-plate method for assaying viableBeauveria bassianapropagules in alfalfa field soil was used to determine fungal inoculum density.B. bassianaeffect on adult populations ofSitona hispiduluswas also investigated by determining the incidence of the fungus in field populations of adults and by exposing adults to known concentrations ofB. bassianain field soil. The 95% CL of inoculum density was between 1,790 and 3,103 colonies per gram of dry soil. The number of infected adults was<10% and did not increase with levels as high as 106conidia per gram of dry soil.