Abstract: Combined effects of Beauveria bassiana and two insect growth regulators (IGR), methoxyfenozide and hexaflumuron, on beet armyworm larvae, Spodoptera exigua (Hübner), were investigated. An obvious synergism effect of the two IGRs with Beauveria beauveria was observed. When the IGR at LC₅₀ concentrations (25×10^-6g/L for methoxyfenozide and 1000×10^-6g/L for hexaflumuron) was mixed with B. bassiana at 4:1 (v/v), the mixtures achieved the best efficacy. The co-toxicity co-efficient (CTC) of methoxyfenozide and B. bassiana was 285, that of hexaflumuron and B. bassiana was 208. Scanning electron microscope observation of the longitudinal section revealed obviously different patterns of the beet armyworm larval integument in different treatment groups.