The susceptibility of Spodoptera exigua(H$ddot{u}$bner) to bifenthrin and chlorpyrifos-methyl in relation to larval development was investigated. Increased tolerances to these insecticides were associated with increasing larval instars. The insecticide tolerance increased linealy with larval body weights in bifenthrin but did exponentially in chlorpyrifos-methyl. Esterase(EST), acetyl-cholinesterase(AChE), and glutathione S-transferase(GST) of different larval ages were analyzed to elucidate variation of insecticide susceptibilities according to larval development within a population. Total EST activity increased linearly with body weights though the specific activities were not varied among ages. Both total and specific activity changes of GST, however, surpassed the rates of body weight gains as the larvae developed. AChE activities was due to the developmental change of its catalytic function. The fifth instar larvae had the lowest catalytic capacity: the highest $K_{m}$ (187.39$mu extrm{M}$) and the lowest $V_{max}$ (0.58nM/min/$mu extrm{g}$). Therefore, different insecticide tolerance of S. exigua according to larval ages can be explained by both enhanced detoxification enzymes and altered AchE.