Monocrotophos has been shown to exert estrogenic effects on teleosts. However, it has not been determined whether monocrotophos itself is the component that is responsible for exerting these estrogenic activities. Therefore, this study was carried out to investigate the estrogenic effects of monocrotophos standard in male goldfish (Carassius auratus), after a 21-day semi-static toxicity test. The induction of vitellogenin synthesis in males exposed to monocrotophos was confirmed with qualitative and quantitative methods. The observed increase in endogenous estrogen levels in males exposed to monocrotophos might explain the induction of vitellogenin. Moreover, we observed a decreased gonadosomatic index, ultrastructure damage of sperm and Sertoli cells, and several alterative testicular enzyme activities in males exposed to monocrotophos, which possibly affect spermatogenesis and male fertility. Our study showed the potential estrogenic effects of monocrotophos standard may act via the induction of vitellogenin synthesis and alteration of testicular ultrastructure and function in male goldfish.