The purpose of this study was to specify the time when individuals are committed to female to male sex change in the protogynous wrasse, Halichoeres trimaculatus, induced by treatment with the nonsteroidal aromatase inhibitor (AI) Fadrozole. In this study, treatment with AI was carried out by providing adult females with a diet containing 500 microg AI/g food for 3 (AI-3), 5 (AI-5), and 10 days (AI-10). We examined the gonadal structure of the fishes histologically at the end of the AI treatment and 30 days after the start of the experiment. At the end of the AI treatment, all individuals in the AI-3 treated group had gonads with degeneration of yolky oocytes, indicating the onset of sex change. Most individuals in the AI-5 treated group had gonads with atretic vitellogenic oocytes, like those in AI-3 treated group, whereas most individuals in the AI-10 treated group had gonads with testicular tissue. At 30 days after the onset of the experiment, approximately 70% of the individuals in the AI-3 treated group had mature ovaries, whereas all fishes in AI-5 and AI-10 treated groups had mature testes, indicating sex change. Therefore, treatment with AI for only 5 days resulted in complete sex change. Our results also indicate that crucial events for testicular differentiation occur within 5 days from the start of AI treatment. Thus, we conclude that females are committed to change into males after 5 days of AI treatment.