The virulence of six isolates ofBeauveria bassiana(Balsamo) Vuillemin to the Russian wheat aphid,Diuraphis noxia(Mordvilko), was bioassayed. All the isolates tested were capable of infectingD. noxia, but their virulence, determined by LC50's and LT50's, varied. An aphid-derived isolate had the lowest LC50, 0.57 × 105conidia/ml, and among the shortest LT50's at all five concentrations of conidial suspension. Two isolates from nonhomopteran hosts and one from an homopteran also displayed significant virulence and were very similar to one another, whereas two Homoptera-derived isolates were significantly less virulent. Log-transformed concentrations and post-treatment time (days) were correlated with probit-transformed mortality induced by each of the isolates. The resulting models, using third-order polynomials, gave good predictions of mortality within the range of concentrations tested. Colony growth rates for four isolates were relatively similar. Differences in virulence among the tested isolates are discussed in conjunction with their geographic origins, insect hosts, and growth and sporulation characteristics.