The bioactivity of the samples of two species of chinaberry,Melia toosendan Sieb. et zucc. and M, azdarach L. collected from different parts of China was assayed with the larvae of imported cabbage worm (Pieris repae L.) .The relationship between the bioactivity and the content of toosendanin analysed by means of HPLC in the samples is discussed. Toosendanin was mainly found in the bark. None or very little toosendanin was found in the leaves and fruits, but they still showed some toxicity to the larvae. Some samples and crude products of toosendanin showed stronger bioactivity as compared with the results from samples containing the same amount of pure toosendanin. This indicates that other than the toosendanin, the constituents in the samples and crude products possess certain bioactivity and possibly some synergistic or antagonistic effects with toosendanin. The ecotypes of chinaberry from different parts of China with regard to their contents of toosendanin and bioactivity is also studied. The chinaberry from Sichuan Province and neighboring region was found to be particularly rich in toosendanin and actively to the insect tested and is worth further investigations. The results of our experiments indicate that an economic, effective and practic method of using the crude products of toosendanin for controlling the imported cabbage worm r.nd other insects is very promising.