Abstract: Diethylstilbestrol (DES) is a typical endocrine disruptor for aquatic animals in the Yangtze River of China. Here we investigated the effects of DES on testicular development and spermatogenesis of fish. Adult male zebrafish (Danio rerio) were used as experimental subjects and were exposed to DES (0.1, 1 and 10 g/L) for 20 days. The histological results demonstrated that the DES exposure led to severe impacts on zebrafish spermatogenesis. To further elucidate mechanisms underlying this phenomenon, we cloned the cDNAs of vasa and dmc1 and analyzed their expression patterns at the tissue and cellular levels. Our results showed that vasa was exclusively expressed in spermatogonia and primary spermatocytes of testis, and that dmc1 was specifically expressed in spermatocytes of testis. Using semi-quantitative RT-PCR we found that DES dramatically suppressed the expressions of dmc1 in a dose- and time-dependent manner, but did not affect the expression of vasa. Moreover, the expression of dmrt1 (the male sex determining gene) and P450 11 (the key enzyme responsible for 11-KT synthesis) were also suppressed by DES exposure. Given that these genes play a role in meiosis and spermatogenesis, we speculated that DES might induce the male germ cell apoptosis in zebrafish by suppressing the expression of dmrt1 and P450 11, and might inhibit the expression of dmc1 which result in impeded meiosis.