Beauveria bassiana(Balsamo) Vuillimen has been identified as a naturally occurring pathogen of larval lesser cornstalk borer,Elasmopalpus lignosellus(Zeller). Studies were conducted to determine the effects of a commercialB. bassianaspore preparation (ABG-6178, Abbott Laboratories) on larvae ofE. lignosellus. B. bassianawas virulent to first and third instars in bioassays in which inoculum was applied to leaf substrates. First instars were more susceptible than third instars. Larvae treated withB. bassianacontinued to develop and consume food at normal rates until they died or pupated. Higher conidial levels were required to cause mortality when inoculum was mixed into the larval soil habitat. Sterilizing soil before bioassaying resulted in a 10- and 1,000-fold reduction in LC50values required to kill first and third instars, respectively.