This experiment was conducted to study the effects of whole yeast (Pichia guilliermondii; CitriStim, ADM, Quincy, IL) cell product supplementation on cecal microflora population and intestinal immune parameters in broilers. In the first experiment, birds were fed 0, 0.1, or 0.2% yeast cell wall product for 42 d. Feeding yeast cell wall products decreased (P = 0.03) the proportion of Escherichia coli in the ceca by 31% compared with the control group. The group fed 0.2% yeast cell wall product had a 20% decrease (P = 0.23) in Salmonella population compared with the control group. In the second experiment, birds were fed yeast cell wall product for 21 d and challenged or not challenged with coccidial oocysts, thus resulting in a 2 (0 and 0.2% whole yeast product) × 2 (coccidial challenge and no coccidial challenge) factorial model. Supplementing whole yeast cell wall product prevented a coccidial infection-induced decrease in the Lactobacillus population (P = 0.09) at 12 d postchallenge. Supplementing yeast cell wall product prevented a coccidial infection-induced increase in the Salmonella population (P = 0.08) and E. coli (P = 0.12) at 12 d postchallenge. At 5 d (P < 0.01) and 12 d (P < 0.01) postcoccidial infection, yeast cell wall product supplementation or coccidial infection increased the regulatory T cell (Treg) percentage in the cecal tonsils, whereas yeast cell wall product supplementation in the coccidial-infected group decreased the increase in Treg percentage. At 5 d postcoccidial infection, coccidial infection increased (P = 0.01) the relative amounts of cecal interferon (IFN)γ mRNA. In addition, the yeast cell wall product supplementation in the coccidial-infected groups further increased (P = 0.15) the IFNγ mRNA. It could be concluded that yeast cell wall product supplementation decreased coccidial-infection-induced increase in E. coli and Salmonella colonization and improved IFNγ mRNA amounts after coccidial infection.