Abstract:This study evaluated the effect of three different concentrations (3x107, 3x106 and 3x105 conidia/ml) of the entomopathogenic fungus, Beauveria bassiana (Bals.-Criv.) Vuill (AUMC 3864) on the adult stage of the tephritid fruit flies, Bactrocera zonata (Saunders), Ceratitis capitata (Wiedemann) and Carpomyia incompleta (Becker). The highest concentration of the fungus (3x107 conidia/ml) caused 100% mortality, 72 hours post adult infection for both of B. zonata and C. capitata, and after 96 hours in case of C. incompleta. The concentration (3x106 conidia/ml) caused 100% mortality after 96, 144 and 120 hours for B. zonata, C. capitata and C. incompleta, respectively. The lowest concentration 3x105 conidia/ml caused 100% mortality of the pest adults after 168, 192, 120 hours for the above metioned species, respectively. Also, the results indicated that, the minimum LT50 values for B. zonata, C. capitata, and C. incompleta were 46.06, 49.40 and 41.63 h after infection by the concentration of 3x107, respectively. Consequently, the most susceptible species to B. bassiana was C. incompleta which presents the least LT50 values (41.63, 69.39 and 76.70 h) after infection by the concentrations of 3x107, 3x106 and 3x105 conidia/ml, respectively. So, this biocontrol agent can be used in controlling fruit flies as a promising tool compared with the conventional insecticides. Keywords:Bioassay, Entomopathogenic fungi, Beauveria bassiana AUMC 3864, fruit-flies, Bactrocera zonata, Ceratitis capitata, Carpomyia incompleta.