The present research represents the first investigations into the development of cross-tolerance in the larvae of Diamondback Moth (DBM) tolerant to Bacillus thuringiensis toxin (Bt-toxin: Dipel^®) and treated with different concentrations of the entomopathogenic fungus Beauveria bassiana (strain BbGHA). Significant differences (at P = 0.05) were obtained between the fungus concentrations used for the treatment of Bt-tolerant and susceptible DBM larvae. The LC₅₀ of the fungus concentrations used for the treatment of Bt-tolerant larvae was 7,700,836.787 (7.7 X 10⁶) conidia/ml versus 133,791.018 (1.3 X 10⁵) conidia/ml for the susceptible larvae. The calculated resistance ratio (RR) was 57.6 times more in the Bt-tolerant larvae compared to the susceptible larvae. Overall, results suggest that Bt-tolerant larval populations of DBM could be controlled by application of B. bassiana, but higher concentrations of the fungal agent than in susceptible larval populations are required. Further research studies are recommended to be done in order to specify the mechanism of this cross-tolerance before exploitation of these results in pest management strategies of this  insect especially when the control is practiced by application of the fungus and Bt-toxin formulations especially with Dipel^®.