Prebiotics are being used as an alternative to antibiotic growth promoters in poultry industry with a variable degree of success. This trial was conducted to investigate the effect of dietary mannan-oligosaccharide (MOS) supplementation on growth performance, selected colonic bacterial population, and immune response in maternally-isolated rock pigeons. The pigeons (n = 36) were randomly distributed in 4 treatment groups (n = 9 per group) with 3 replicates or pens (n = 3) in each group. Birds were fed either a corn/wheat-based basal diet (control group/CON) or the same diet supplemented with 0.1%, 0.2%, or 0.5% MOS for 35 d. On d 35, birds were killed to collect visceral organs, colonic contents, and serum. Colonic contents were used to enumerate selected microbiota and serum was used to detect antibody titer against the Newcastle disease virus. Cell-mediated immunity was determined by measuring the skin thickness following 2,4-dinitrochlorobenzene challenge. Results showed that supplementation did not affect the BW of birds. During wk 4, feed intake was significantly higher in the 0.2% (187.9 ± 0.86) and the 0.5% (186.4 ± 0.86) MOS-supplemented groups compared with the CON group (180.7 ± 0.86). Gizzard weights (with and without digesta) were significantly higher in the MOS 0.1% (10.67 ± 0.33 and 8.22 ± 0.26) and the MOS 0.2% (9.91 ± 0.33 and 7.94 ± 0.26) groups compared with the CON group (7.56 ± 0.33 and 6.25 ± 0.26). Small intestinal weight was significantly higher in the MOS 0.2% group (14.71 ± 0.56) compared with the CON group (9.56 ± 0.56). Lengths of small intestine (MOS 0.1% = 92.56 ± 1.69, MOS 0.2% = 90.79 ± 1.69, MOS 0.5% = 90.57 ± 1.69) in all the MOS-fed groups and large intestine in the 0.1% (3.50 ± 0.02) and the 0.5% (3.47 ± 0.02) MOS-fed groups were significantly higher than the CON group (small intestine = 77.63 ± 1.69, large intestine = 2.41 ± 0.02). Weights of heart, liver, pancreas, and large intestine remained unaffected. Feeding of MOS did not influence colonic population of Escherichia coli, Clostridia, and Bifidobacteria. Lactobacilli count was significantly higher in the 0.2% MOS-fed group (9.77 ± 0.12) compared with the CON group (9.19 ± 0.16). Mannan-oligosaccharide did not affect the immune response of the birds as antibody titer against the New Castle disease virus and the cell-mediated immunity remained similar in all the treatment groups. In conclusion, dietary MOS supplementation influenced only the colonic lactobacilli population without any apparent effects on the production performance in growing pigeons.