Sex control in aquaculture aims to benefit from desirable traits expressed differentially by each sex. In ornamental fish, such manipulation can be profitable, especially in species with high price discrepancy between sexes. This is particularly true for the fancy males of Siamese fighting fish (Betta splendens Regan, 1910), which are very popular aquarium pets. Therefore, fry originating from controlled reproduction were fed initially with artemia nauplii from day 4 post-hatch up to day 7 post-hatch. Subsequently, they were orally administered with 17a-methyltestosterone (MT) (via inert feed) in four concentrations (1, 2, 3, and 4 mg/kg of hormone in feed) for eight weeks. The experiment was repeated with fry of the same origin in identical conditions with the exception of a prolonged supply period (from day 4 post-hatch until day 11 post-hatch) of artemia nauplii enriched with red pepper. The results demonstrated that the 3 and 4 mg/kg doses resulted in 100% masculinization and the extended provision period of enriched artemia nauplii increased the survival rate of the offspring. However, mortality rate increased in all hormone treated groups in both experiments, partly due to the prolonged hormone administration period. Therefore, oral administration of low MT doses is effective for the masculinization of Siamese fighting fish.