As a selective biological insecticide1 spinosad has been used widely for the control of pests including beet armyworm, Spodoptera exigua (Hübner) (Lepidoptera: Noctuidae). To form effective pest control strategies, lethal and sublethal effects should be considered for a complete analysis of spinosad impact. However, few studies have been reported to investigate sublethal effects of spinosad on S. exigua. This study attempts to evaluate the lethal and sublethal effects of spinosad on this pest by recording and analyzing various toxicological and physiological parameters. The toxicity of spinosad against S. exigua was determined under laboratory conditions by oral exposure of late second-instar larvae to the compound. The LC₅₀ values of spinosad to S. exigua at 48 and 72 h after treatment were 0.317 and 0.293 mg·kg^-1, respectively. Spinosad at sublethal concentrations significantly extended the developmental period of survivor larvae, and reduced larval wet weight. Postexposure effects were indicated by decreased pupation ratio and pupal weight, by prolonged prepupal and pupal periods and by decreased emergence ratio, fecundity and longevity of adults. The net replacement rate (R₀) tended to be lower in the exposed spinosad groups than those in the unexposed spinosad group. Intrinsic rate of population increase (r_m) for the high-dose group (0.365) was significantly lower than the control (0.521) and the low-dose group (0.521), but the latter two were not significantly difference. These results suggest that the combination of lethal and sublethal effects of spinosad might affect S. exigua population dynamics significantly by decreasing its survival and reproduction, and by delaying its development.