The western tarnished plant bug, Lygus hesperus Knight, is susceptible to the naturally-occurring pathogen, Beauveria bassiana (Balsamo) Vuillemin, in the San Joaquin Valley of California. Recent research efforts have focused on selection of Beauveria strains that were effective against Lygus under the high-temperature conditions typical of the cotton (Gossypium spp.) production season. However, the most appropriate use of this pathogen may not be as a rescue treatment. Alternatively, B. bassiana may be useful to efforts to target overwintering populations of Lygus if isolates are available that are highly virulent under the low temperature conditions typical of winter and early-spring in the San Joaquin Valley. One commercially-available isolate and 4 native isolates of B. bassiana were assayed against L. hesperus adults under constant temperatures of 12.8, 18.3, and 23.9°C. Although decreasing temperatures were associated with diminished Beauveria-induced mortality of Lygus and slower development of disease symptoms, no differences in efficacy were detected among the tested isolates. Differences in the patterns of occurrence of Beauveria disease symptoms were observed among isolates at some temperatures, but those differences were not substantial. Furthermore, results at some temperatures suggested potential influences of Lygus adult age or gender on susceptibility to B. bassiana. Those effects should be further investigated. Overall, the results did not indicate that any of the tested isolates of B. bassiana were superior to the commercially-available isolate under low temperature conditions.