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¹ Afyon Kocatepe University, Department of Medical Biology and Genetics, Faculty of Veterinary Medicine, 03200 Afyonkarahisar, Turkey DOI : 10.4194/1303-2712-v17_2_06 Viewed : 4601 - Downloaded : 3773 Exogenous steroids could change the direction of sex in lower vertebrates including fish. Therefore, this study is conducted to detect the effects of 17ß-estradiol on the sex determination and differentiation in fathead minnows. The fathead minnow larvae were exposed to 0 (control), 0.5 or 1 μM 17ß-estradiol for different periods during early development (Day 5 fish post spawning to Day 25 fps) and then reared in glass aquaria to adult age, which was Day 150 fps. At the end of experimentation, tissue samples were collected. Gonadal abnormalities were determined by gross-morphological and histological examinations. Our results showed that 17ß-estradiol induced developmental abnormalities in the gonads of both sexes and the skewed the sex ratio toward male in a dose and exposure time dependent. Most important findings of this study was that 17ß-estradiol clearly induced paradoxical sex differentiation from female to male but perhaps non-functional masculinization in fathead minnow. Keywords : Sex differentiation, paradoxical masculinization, gonads, 17ß-estradiol, fathead minnow