Earthworms (Eisenia fetida) as subject in the test are exposed to chlorpyrifos of a sub-lethal dose for 8 weeks in artificial soil．Effects of the exposure on SOD, CAT, and GSH-Px activities in earthworms were observed. Results show that the compound suppressed the activities significantly. So the enzymes of the anti-oxidizing defense system of earthworms can be cited as indicators of chlorpyrifos toxicity. Besides the commonly used ecological toxicity indexes, such as LC₅₀, oviposition rate and hatching rate, the activities of the antioxidases, were closely related to the toxic effect of the pesticide. However, the enzymes varied in sensitivity to the chemical． SOD and GSH-Px were more sensitive than CAT．Therefore, in diagnosing ecological toxicity, it is advisable to have different indices form a mutually supplementary indicator system to improve the sensitivity and efficiency of pollution identification and long term diagnosis．