Abstract: Bleached kraft pulp mill effluent(BKME), which has been reported to be environmental estrogenic substances, may cause threats to aquatic species. In this study,the toxicity of BKME and the effect of BKME on plasma sex steroids (testosterone and 17β estradiol) in carp ( Carassium auratus) were determined under laboratory conditions. The results of toxicity test showed that 96 h and 168 h LC₅₀ values associated with 95% CI were 439.0±30.9 ml/L and 296.6±62.0 ml/L respectively. It was found that there was no significant effect on plasma testosterone levels when carp were exposed to different concentrations of BKME (40,80,100 and 120 ml/L) for 30 days. However, exposure to BKME resulted in a marked decrease in plasma 17β estradiol level in carp. It is suggested that BKME could induce depressions of plasma sex steroids in fish, thus threatening reproduction of fish.