Effect of fluctuating temperatures on the development of the immature beet armyworm, Spodoptera exigua (H bner), was analyzed. At constant rearing temperature regimes, the estimated developmental threshold temperatures were varied among stages and instars, but had an average 13$^{circ}C$ from egg hatch to adult emergence. Based on the 13$^{circ}C$ threshold temperature, we set up three different rearing temperature regimes having the same day-degrees. Two fluctuating temperature regimes changed significantly the developmenta1 period expected by the constant rearing temperature regime. Under the same thermophase temperature (25$^{circ}C$), the thermocycling regime with the higher cryophase temperature (10$^{circ}C$) decelerated the developmental rate probably by lowering temperature limit thor development, but that with the lower cryophase temperature (5$^{circ}C$) gave a negative developmental effect.