Effects of dietary tetradecylthioacetic fatty acid (TTA) on muscle fat, development of gonads and early sexual maturation in S0 Atlantic salmon during the first year in sea were investigated. TTA (0.5% w/w) was added to the feeds for 8 weeks in the spring. In May, at the end of the TTA-feeding period, the fish in the TTA group had significantly (P < 0.05) less fat (10.1%) stored in muscle compared with the control group (10.8%). In September, mean male gonadosomatic index (GSI) in maturing fish in the TTA group was found to be lower compared with the maturing fish in the control group (P = 0.05). On the basis of GSI values, male sexual maturation in September was 10.0% vs. 14.4% for the TTA and the control group respectively. Thus, relative to the control group, the incidence of male sexual maturation in the TTA group was reduced by about 1/3 (P = 0.002). Production data was not affected by dietary supplementation of TTA. This study reveals that TTA significantly reduces the incidence of male sexual maturation in S0 Atlantic salmon. A significant elimination model of TTA in fish muscle that takes into account, the growth rate of the fish was further developed in this study.