Technical Abstract: As a synthetic androgen, 17 a-methyltestosterone (17MT) is frequently used to redirect the course of sex differentiation by exposing the undifferentiated gonad at sufficient dosage. This hormone has been widely accepted as a safe and effective treatment for sex-reversal in many fish species, and it is administered to larval tilapia (3 – 12 days old) for approximately 28 consecutive days to produce populations of >90% males. This study assessed the safety of 17MT-treated feed when administered to larval Nile tilapia Oreochromis nilotica at 1-, 3- and 5-times the proposed dosage of 9 mg 17MT kg fish d-1 for 28 consecutive days. Despite elevated ammonia levels measured during the last 5 d of the study, environmental conditions were acceptable for rearing tilapia. Fish fed aggressively regardless of the concentration of 17MT in the feed, behavior was considered normal with no dose-related differences detected and no mortality was observed in the 3 x group. Fish that were treated with 5-times the proposed therapeutic dosage had significant pathological changes. Based on the results of this study, the 17MT margin of safety extends to at least 3 x (27 mg 17MT kg fish-1 d-1) the proposed dosage of 9 mg 17MT kg fish-1 d-1when administered in feed for 28 d to tilapia.