Polychlorinated biphenyls (PCBs) are nondegradable pollutants that are known to affect physiology of organisms. To assess the physiological alteration of the pollutants on insects, the beet armyworm, Spodoptera exigua, was used as a model insect because of its wide-range occurrence and high biochemical detoxification against toxic chemicals. Low ppb levels of PCBs applied on newly laid eggs of S. exigua were found to inhibit egg development and later larval development. Larvae fed with PCB-contaminated artificial diet (below ppb levels) exhibited significant mortalities, though it did not affect the capacity of diet consumption or digestibility. PCBs injected into larval hemocoel of S. exigua were not excreted. In vitro assay showed that PCBs induced some fat body esterases of S. exigua. However, PCBs did not affect juvenile hormone esterase activity in larval hemolymph of S. exigua. These results suggest that the immature stages of S. exigua are highly susceptible, but responsive to PCBs by inducing some esterases.