The effects of three inclusion levels of mannan oligosaccharides (MOS) derived from the outer cell wall of a select strain of Saccharomyces cerevisiae (Bio-Mos, Alltech Inc, USA) on growth, feed utilization, body composition, and antioxidant enzyme activity of tilapia (Oreochromis niloticus) were determined. Specimens (12 g) were fed a commercial diet supplemented with 0 parts per thousand, 2.5 parts per thousand, 3.5 parts per thousand, or 4.5 parts per thousand dietary MOS for 60 days. Food conversion rates (FCR), specific growth rates (SGR), and the biochemical composition of muscle tissue were determined. Growth was greatest, protein was highest, and lipid was lowest in fish fed the 2.5 parts per thousand MOS diet. The FCR was significantly better in all MOS-treated groups than in the unsupplemented control. At the end of the study, antioxidant enzyme activity of superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GSH-Px) in liver and muscle tissue was generally higher in the supplemented diets than in the control and significantly higher in the 4.5 parts per thousand treatment. Oxidative damage due to lipid peroxidation (LPO) was assessed by formation of malondialdehyde (MDA) and protein carbonyl (PC), both of which were significantly lower in liver tissue in all MOS-supplemented diets than in the control.