Soil inoculations with the entomopathogenic fungus Beauveria bassiana (Balsamo) Vuillemin were made to identify optimal application strategies against overwintering populations of the Colorado potato beetle, Leptinotarsa decemlineata (Say). Tillage of conidia into the soil enhanced fungal persistence, whereas conidia applied to the surface persisted poorly. Despite generally good persistence, soil applications did not reduce the emergence of overwintering adult beetles, in part because of cool, dry soil conditions. Significant mycosis (up to 57%) was achieved only against postemergent beetles in treatments with high inocula densities in late spring. Efforts to reduce populations of overwintering adults with Beauveria must be delayed until spring emergence has peaked and environmental conditions have moderated.