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Though an experiment of exposing Daphnia magna and recovering its offspring for both 21 days, the chronic toxicity of perfluorooctane sulfonate (PFOS) to Daphnia magna and its first and third broods F1 (1st) and F1 (3rd) were investigated. Results show that when the mass concentration of PFOS is higher than 20 mg/L, the number of offspring, length and innate increase rate are significantly inhibited. Compared with the control group, the reduction of the three indicators under the highest exposure concentration are 62.2%, 40.6%, 34.0% (p<0.01), respectively. The toxic effect becomes more serious with gradually increasing of exposure concentration, which shows an obvious dose-response relationship between PFOS concentration of exposure and the toxicity to Daphnia magna. The number of offspring and length of the F1 (1st) generation are significantly inhibited when PFOS mass concentration is higher than 30 mg/L. The F1 (3rd) generation's recovery extent is better than that of the F1 (3rd) generation, and all the indicators recover closely to those of the control group except that the number of offspring, length and innate rate of increase are significantly inhibited when the Daphnia magna is exposed to the PFOS of 50 mg/L. As the most sensitive indicator, the number of offspring, length and innate rate of increase of Daphnia magna are suggested for evaluation of PFOS chronic toxicity. [ABSTRACT FROM AUTHOR]