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Previous studies have reported that triclosan (TCS) could possess an androgenic activity.We aimed to investigate the effects of TCS on gonadal differentiation and development in the frog Pelophylax nigromaculatus,a sensitive amphibian species to androgenic chemicals.P.nigromaculatus tadpoles at stage 24 were exposed to TCS (3,30,and 300 nmol/L) to stage 46 in a semi-static exposure system.At the end of exposure,gonadal morphology and histology,sex ratio and gonadal expression of sex-biased genes were examined in P.nigromaculatus.In each TCS treatment group,we found several individuals whose gonads exhibited morphological and/or histological abnormalities.Gonadal histological abnormalities were characterized by few oocytes and many somatic cells.Although the percentage of the individuals with abnormal gonads was low (7.8％) among all animals treated with TCS,statistical test revealed the sex ratios in the 3 and 300 nmol/L TCS treatment groups were significantly different from the solvent control.In the 30 nmol/L TCS treatment group,abnormal gonads were also observed,although the sex ratio was not changed compared with the solvent control,which was possibly due to the smaller sample size in this group.In all the TCS treatment groups,the sex ratios were not obviously male-biased,but the expression levels of some sex-biased genes were significantly altered by TCS.Altogether,our results suggest that TCS,even at environmentally relevant concentrations,could disrupt gonadal differentiation and development in P.nigromaculatus,but we are not sure whether the disrupting effects were associated with masculinization or feminization.