Nile tilapia farming using monosex male population has been reported to improve the productivity by 10% compared to that of mix-sex culture. This study aimed to determine an immersion dose of 17α-metiltestosterone (MT) that resulted in higher male percentage, growth, survival, and to measure testosterone level in the fish body. The experiment compared three MT immersion doses, namely: 0; 1.8; and 5.4 mg/L, each with three replications. Immersion was conducted to 14-days-old larvae for four hours, with a density of 100 fish/L of water. Testosterone levels in fish was measured using ELISA method, and sex identification was performed by histological method. The result showed that the percentage of male fish was the same in treatments 1.8 mg/L and 5.4 mg/L, which were 53-65% higher than that of the control without MT treatment. Fish growth and survival until day 56 post immersion were not significantly different amongst treatments. The results of ELISA analysis showed that the levels of testosterone in larvae just after immersion was similar in 1.8 mg/L and 5.4 mg/L treatments, which then drastically decreased on day 14 after immersion. On day 28 after immersion the levels of testosterone of the MT treatments were similar to that of the control, i.e. about 2 ng/g. On day 71 the testosterone levels in sex reversed and normal males fish remained the same at about 0.3 ng/g (P>0.05). By PCR method with specific primer, sex reversed and normal males could be distinguished. In conclusion, sex reversal by immersion at a dose of 1.8 mg/L can be considered as a standard protocol for Nile tilapia sex reversal. Testosterone level in the body of MT-treated fish could be reduced to the same level as the control in less than one month post immersion.Keywords: sex reversal, 17α-methyltestosterone, testosterone levels, immersion, tilapia ABSTRAK Budidaya ikan nila menggunakan sistem monoseks jantan telah dilaporkan dapat meningkatkan produktivitas hingga 10%. Penelitian ini dilakukan menentukan dosis perendaman hormon 17α-metiltestosteron (MT) yang menghasilkan rasio ikan jantan tertinggi, pertumbuhan, sintasan, dan mengukur kadar testosteron dalam tubuh ikan nila. Dosis hormon MT yang diuji adalah 0; 1,8; dan 5,4 mg/L. Setiap perlakuan dosis diberi tiga ulangan. Perendaman dilakukan selama empat jam, dengan jumlah larva (umur 14 hari pascatetas) 100 ekor dalam 1 L air. Hasil analisis histologi menunjukkan bahwa persentase ikan nila jantan adalah sama pada perlakuan 1,8 mg/L dan 5,4 mg/L, dan 53-65% lebih tinggi daripada kontrol tanpa direndam MT (54% jantan). Dengan analisis ELISA, kadar testosteron dalam tubuh larva sesaat setelah perendaman MT dosis 1,8 mg/L dan 5,4 mg/L adalah sama, tetapi turun drastis pada hari ke-14 pascarendam, kemudian kadarnya menjadi relatif sama pada hari ke-28, yaitu 2 ng/g). Pertumbuhan bobot dan sintasan ikan sampai hari ke-56 adalah sama. Dengan menggunakan metode PCR, jantan fungsional dan jantan normal dapat dibedakan, dan pada hari ke-71 kadar testosteron ikan jantan fungsional dan jantan...