In this study, the effects of four Beauveria bassiana isolates, obtained from the soil of Fashand, Atashgah and Ghare-agaj and one from a grass hopper Sphingonotus sp. on the adults of Russian wheat aphid Diuraphis noxia were studied under laboratory conditions. After determination of the minimum and maximum concentrations, the adult Russian wheat aphid was treated with conidial concentrations of 1×103 to 1×10 6 spore/ml to calculate LC50 & LC90. DEBI002 proved to have the lowest LC50 (4 ×105 spore/ml) whilst DEBI010 showed the highest LC50 (2.5×106 spore/ml). The lowest LT50 was obtained 2.8 days for DEBI002 at 1×10 6 spore/ml. In a completely randomized block design, the mortalities caused by the isolates on 6th & 10th days were as follows: At 1×10 5 spore/ml, on 6th days DEBI002(34.95% ± 3.64), DEBI015(37.1% ± 6.11) & DEBI001 (32.35% ± 1.3) were classified as level A & DEBI010 (18.74% ± 1.91) as level B. On 10th days, at 1×10 5 spore/ml DEBI002(45.1% ± 1.23), DEBI015(39.48% ± 7.21) and DEBI001(39.43% ± 2.65) were classified as level A & DEBI010 (16.23% ± 3.9) as level B (Duncans Muitple Range Test).