The histological structural changes in the testis and liver tissues of the swordtail fish (Xiphophorus helleri) exposed to 50 μg/L 17β-estradiol (E₂) for 50 days were investiged. The results showed that, after exposure to E₂, the number of spermatogonia was reduced, the growth and development of spermatogonia were stagnated, and the number of sperm was decreased. The liver cells exhibited fat vacuoles and fat deposition, indicative of fatty liver symptoms. The results indicated that E₂ had strong estrogenic effects. The extent of testis and liver tissue damage changed with E2 exposure time. The longer the exposure, the more serious the damage was. But the damage was still within the tolerance of the fish, without causing its death. Therefore, E₂ exposure damage to the testis structure can be used as a histological biomarker for detection of environmental estrogens.