'This study was concerned with the efficiency of the entomopathogenic
fungus Beauveria bassiana (Balsmo), on the mature larvae (4^th instar) of the potato
tubermoth Pnthonmeee opercu/ella (Zeller) with two different environmental factors
under laboratory conditions, e. g. temperature regimes (18, 21, 24, 27 and 30°C) and
relative humidity regimes (50, 60, 70_a 80 and 90% R. H.). Four concentra-tions of conidial powder were prepared (1x10, 10⁷, 10⁶ 10⁵) conidia/ml. Some biological aspects were determined, e,g., Survived larvae, pupation and emergence percentages, Results indicated that there were a positive relation between fungal concentrations and reduction inpercentages of larval development. Different regimes in temperature and relative humidity have great influence on the fung~1 pathogenicity. Highly significant reduction 'in the fourth larval instar ' development was' obtained with 27 and30°Ctemperature regimes and with 80 and 90% relative humidity regimes. Percentage of larval mortality was scored with the four fungal concentrations for 8 days, It was, clearly noticed that 10⁸ fungal concentra- tion gave 60,67' and 81.67 percentage of mortality after 3 and 4 days, respectively. When ,the: fourth I€lnial instar was reared on gam'ma, irradiated potato tubers (40, soano 120 Gy) the larval mortality' was significantly' lncreased especially with 120 Gy. larval duration was increased as the dose of gamma irradiation increased.