To perform the purpose of this study of evaluate the effects of mannanoligosaccharides on nutrients digestibilities, faecal microbiota composition and fermentation end-products, and immunological parameters in adult health dogs, eight adult beagles were used in two 4 X 4 Latin square design. A 15-d adaptation phase preceded a 5-d total collection of feces for digestibility trial and 1-d (day 21) collection of fresh fecal samples for bacterial enumeration of total anaerobes, total aerobes, Bifidobacterium, Lactobacillus, Clostridium and E. coli. Additional feces aliquots were used for pH measurement and determination of biogenic amines (pool of ten amines), short chain fatty acids (butyrate, propionate and acetate) and lactic acid. Also on day 21, blood sample was collected for immunophenotypic quantification of lymphocyte subsets CD4/CD5+, CD5+, CD8/CD5+, CD45+, CD45/CD21+ by flow cytometry. Dogs were fed individually calculated amounts of four high protein and low fiber dry extruded isonutrient diets, balance for maintenance (AAFCO, 2004). Experimental diets had four inclusion levels of purified yeast (Saccharomyces cerevisiae) cell wall (YCW), as fed basis: 0%, 0.15%, 0.30% and 0.45%. Means were compared by Tukey test, and polynomial and orthogonal contrasts (p<0.1), utilizing Proc GLM of SAS software. Digestibility of dry matter, crude protein, acid hydrolyzed fat, nitrogen free extract and crude energy did not vary between diets, showing no effects of YCW. Yeast cell wall supplementation did not result in differences in fecal microbial populations (log of cfu per g of feces DM). However it was found a linear increase in fecal concentration of butyrate (mMol/kg DM; p=0.055), and a linear reduction of fecal concentrations of tyramine (mg/100g feces; P=0,1), histamine (P=0,07), and a quadratic reduction of phenylethylamine (P=0,07) and tryptamine (P=0,07), suggesting alteration in bacteria's metabolic activity and end-products. Dogs showed a linear increasing of CD5+ lymphocyte subset concentration (cells/μL; P=0,1) and a higher number of CD45/CD21+ (cells/μL; P=0,053) with YCW addition. Yeast cell wall addition between 0.15 to 0.45% of diet did not interfere in digestibility, and fecal quality. At this dosage the prebiotic was not effective in changing concentrations of fecal microbial populations, however, interesting changes was found on bacterial end-products. The decreasing in undesirable amines and increasing in butyrate concentration may be important to the animal, suggesting a better bacterial activity in the gut. Immunophenotypic evaluation evidenced improvement of dogs' immunological status, demonstrated after 21d of yeast cell wall consumption.