The pejerrey Odontesthes bonariensis is a teleost with a strong temperature-dependent sex determination system (TSD). Dmrt1 and Cyp19A1 genes have been implicated in the sex differentiation process in fish, but little is known about their behavior in TSD species. In this context the expression pattern of both genes was studied during sex differentiation at feminizing and masculinizing temperatures. Both genes showed differential expression in relation with the temperature and consequently the inducible sex, supporting the idea that they are involved in sex differentiation in this species.