Dietary mannanoligosaccharide (MOS) from commercial product, Bio-Mos supplementation, has been examined for its effects on weight gain and feed conversion of domestic mammals and birds, but very few studies have evaluated the responses of aquacultural species to MOS. A feeding and digestibility trial was performed to asses the potential beneficial effect of two levels of Bio-Mos on growth, feed utilization, survival rate and nutrients' digestion of gilthead sea bream (Sparus aurata) with an initial average weight of 170 g. Bio-Mos was added at 2 or 4 g kg(-1) to a fish meal-based control diet, and each diet was fed to triplicate groups of 1-year-old gilthead sea bream. After 12 weeks, there were no differences in survival rate among fish fed experimental diets (P > 0.05). It was observed that a significant improvability existed for both growth and feed utilization in fish fed diets supplemented with Bio-Mos (P < 0.05). Body proximate composition remained unaffected by Bio-Mos supplementation in fish fed experimental diets (P > 0.05). Apparent digestibility values for protein, carbohydrate and energy were appreciably affected by the inclusion of two different levels of Bio-Mos, only lipid digestibility was the exception. In conclusion, the results of this trial indicate that 2 g kg(-1) dietary supplementation with BIO-MOS seem to be most positive for gilthead sea bream production.