This is the first study on the effects of dietary mannan oligosaccharides (MOS) on growth, body composition, and intestine and liver histology of hybrid tilapia (Oreochromis niloticus x O. aureus). Experimental diets were prepared from commercial trout diet, supplemented with MOS at levels of 0, 1.5, 3.0, or 4.5 g MOS/kg feed and randomly assigned to triplicate groups. At the end of the trial, there were no significant differences between treatment groups (p>0.05) in growth parameters (live weight gain, specific growth rate, feed conversion ratio, protein efficiency ratio) or body indices (hepatosomatic and viscerosomatic). Dry matter and protein contents increased with increasing rates of dietary MOS (p<0.05) while the mean villi length of fish fed the diet containing 1.5 parts per thousand. MOS was significantly longer (p<0.05) than that of the fish fed 4.5%. dietary MOS. The different levels of dietary MOS had no detrimental effects on liver tissue or general fish health.