The present study aimed to evaluate the effect of mannan-oligosaccharides (MOS) on the body weight gain, feed intake and feed conversion rate of Guinea pigs (Cavia porcellus) during the fattening period. Seventy five male Guinea pigs (28 ± 3 days of age) were randomly allocated in three experimental treatments applied as additives to a common basal diet as follows: (a) Control (no additive), (b) Growth Promoter Antibiotic (APC) (additive Zn-Bacitracin 0.1 g/kg), and (c) MOS (additive mannan-oligosccharide, 0.5 g/kg). The experimental period was six weeks. No significant differences between treatments and the control were observed.