Maintaining gut health and well balanced intestinal microflora is important for the profitable production of high quality poultry. We investigated the impact of mannanoligosaccharides (MOS) in comparison with flavomycin on turkeys performance and intestinal colonization by harmful and beneficial microflora. Two hundred forty turkeys (four replicates of 12 birds per treatment) were fed on diet supplemented with flavomycin (8 mg/kg feed) or with low (1g/kg feed during the entire study), medium (4 and 2 g/kg feed in the first and second 8-week periods, respectively) and high (10 and 4 g/kg feed in the first and second 8-week periods, respectively) level of MOS. Basal diet without supplements was applied as control. After 16 weeks of feeding, the animals were sacrificed and microbial populations (Bifidobacterium, Lactobacillus, streptococci, Escherichia coli and other enterobacteria) attached to the colonic epithelium were assessed. Moreover, colonic epithelia were washed to remove the bound microorganisms and examined for their susceptibility for colonization by exogenous probiotic and pathogenic strains in vitro. In all experimental groups the reduction of the bacteria attached to the epithelium was found, except for increase of streptococci in flavomycin-receiving group and bifidobacteria in flavomycin and MOS-receiving groups. Higher doses of MOS had stimulating effect on body weight gain. In vitro studies revealed a great potential of MOS to reduce attachment of enterobacteria and in higher doses to enhance adhesion of Bifidobacterium and selected Lactobacillus strains (P<0.05).