SummarySupplementation with Saccharomyces cerevisiae (SC) in horses may have some potential to modify microbial populations and thereby improve fibre digestibility. The aim of this study was to investigate the effects of SC on apparent total tract digestibility (ATTD) of nutrients, with a special focus on fibre digestion in healthy horses. The fermentation profile of microbial populations was another focus of interest. Twelve geldings were randomly assigned to three groups. The basal diet consisted of cracked corn (2 g starch/kg body weight [BW]) and hay (1.2 kg/100 kg BW). During adaptation (3 weeks) and the total faecal collection period (5 days), cracked corn was fed once daily either as control (0 g SC) or supplemented with 1 or 3 g SC (1 g SC = 2 9 10 10 colony-forming units [cfu]). There was a 4-week wash-out period between the different SC regimes. Faeces were sampled by rectal collection for the analysis of pH, nitrogen, lactic acid, short-chain fatty acids (SCFA) and SC. In faeces, mean SC cfu was significantly lower than the quantity supplemented: 0 g SC, no detection; 1 g SC, 1.1 9 10 6 cfu; 3 g SC, 3.6 9 10 6 cfu. Apparent total tract digestibility of crude fibre varied approximately 40%without any treatment-related effects. Short-chain fatty acids, lactic acids and pH in faeces were not significantly affected by SC supplementation. Saccharomyces cerevisiae supplementation was not associated with any changes in the fermentation profiles, and fibre digestion accordingly remained unchanged in intact and healthy horses. Taking into account that fibre digestion remained unchanged and recovery rate of SC in faeces was mariginal, colonization and proliferation of SC in the healthy equine intestinal tract seems to be unlikely.