Abstract: An experiment was carried out to explore acute and chronic toxicity of nonylphenol ethoxylates (NP₄₀EO) to Daphnia magna and the chronic effects of the substance on fatality, growth and reproduction of Daphnia magna after 21 days of exposure, in an attempt to establish a dose-response relationship between the two and search for sensitive biological indices. Results of the acute toxicity test show that the 48 h-EC₅₀ of NP₄₀EO to Daphnia magna was 14.23 mg·L^-1. Results of the chronic toxicity test show that NP₄₀EO had significant effects on the average number of molting, the first reproduction time, the total number of eggs per female, the number of brood per female and the average body length of survivors of the Daphnia magna. Among them, the exposure concentration of 0.445 mg·L^-1 could significantly reduce the number of molting, the average number of brood, the total number of eggs per female, and the average body length. Therefore, it could be concluded that after overall consideration of all the indices that the lowest observed effect concentration (LOEC) of NP₄₀EO for Daphnia magna is 0.445 mg·L^-1, and the no observed effect concentration (NOEC) is 0.11 mg·L^-1. In addition, the study has also found that the first molting time and the first reproduction time of Daphnia magna were significantly affected by NP₄₀EO 1.78 mg·L^-1 in exposure concentration. All the findings demonstrate that NP₄₀EO has some inhibitory effect on growth and reproduction of Daphnia magna.