Abstract: Experiments were designed to rear Spodoptera exigua in laboratory and to establish a bioassay method evaluating the toxicity of Bt against S. exigua. Results showed that artificial diet could be used to rear S. exigua successfully. The development, rate of pupation, emergence and sexratio of S. exigua were normal. Factors such as developmental stage of the testing larvae, length of incubation period afer infection and temperature would affect LC₅₀ values. It is recommended that newly hatched larvae should be used in bioassay and reliable and consistan tresults can be abtained 7 days after infection at 25℃.