In high energy-demanding horses, replacing cereals by sugar beet pulp (SBP) rich in soluble fiber may be of interest to limit hindgut dysbiosis and subsequent mucosal inflammation. This study aimed at evaluating the effect of partial substitution of cereals with SBP on the hindgut microbial ecosystem and the mucosal associated immunity. Six adult exercised Trotteurs Français were enrolled in a 3 × 3 Latin-square design with experimental periods (EP) and wash-out of 21 days each, to test 3 isoenergetic diets distributed in 2 equal meals. On a dry matter (DM) basis, the high-fiber diet (HF) was composed of 100% hay, the high starch diet (HS) of 55% hay and 45% barley, and the high-soluble fiber diet (HSF) of 50% hay, 21% barley and 29% SBP. Horses received hay ad libitum during the wash-out periods. Barley and SBP were introduced gradually during the five first days of each EP. Feces were collected on the last day of each EP to enumerate total anaerobic and cellulolytic bacteria and to measure volatile fatty acids (VFA), lactate, secretory IgA (SIgA) concentrations, DM and pH. Effects of diet, period and their interactionwere tested using MIXED procedure (SAS). Horses fed HS had significantly higher total anaerobes concentration and tended to have lower cellulolytics concentration, confirming hindgut ecosystem alteration with HS (Table 1). No significant diet effect was observed on SIgA concentration. With HSF, cellulolytics counts and total VFA concentrations were higher compared to HS, emphasizing increased fiber hindgut fermentations. In line with the higher amount of highly fermentable fibers in this diet, VFA concentrations were also higher compared to HF. These results suggested that replacing part of cereals by SBP can help limiting hindgut dysbiosis while maintaining a high energy supply through soluble fiber fermentation. Analyses of bacterial diversity and structure, and of blood VFA concentrations would provide further explicative information concerning SBP interest.