The migration of the primordial germ cells (PGCs), formation of the genital ridge, gonad sex differentiation, and their variations under different water temperatures were investigated using histological methods in the teleost, Pseudobagrus vachelli. The results showed that on day 1 the PGCs were at the splanchnic mesoderm above the gut. On day 5, PGCs migrated to the peritoneal epithelium. The genital ridge was formed on day 8 and lacked PGCs. The primordial gonads were formed on day 14. On day 23, the primordial gonads differentiated into ovaries or testes. From day 10 on, the larvae were cultivated in different water temperatures (20±0.5℃, 24±1.0℃,30±0.5℃, 34±0.5℃) for 25 days. The control group was cultivated at natural water temperature (24±1.0℃). The sex ratio and percentage of male larvae were calculated after the experiment. At natural water temperature (24±1.0℃) and lower water temperature (20±0.5℃), the sex ratio was approximately 1∶1 (1∶1.09 and 1.22∶1 respectively). However in the group cultivated at 30±0.5℃, the sex ratio was 1∶4.89 and the percentage of males was 83.3±0.7%. The sex ratio was 2.85∶1 and the percentage of males was only 26.4±0.4% in the group cultivated at 34±0.5℃. These results suggest that water at 30±0.5℃ influenced the larval gonad towards differentiating into a testis, and water at 34±0.5℃ gave the opposite result, i.e. the gonad differentiated into an ovary. This experiment showed that the sex determination of P. vachelli is temperature-dependent sex determination.