Fiber digestion was characterized in 8 yearling geldings (mean initial body weight, 341 kg) fed 4 typical diets differing in ingredient composition, with and without a commercial product that contained microbial xylanase and cellulase. The experimental design was a 4 × 4 Latin square with repeated measures in which horses were fed Coastal Bermudagrass hay at a rate of 1.5% of body weight daily (as-fed basis) plus sufficient textured sweet feed, pelleted concentrate, whole oats, or alfalfa cubes to meet National Research Council requirements for digestible energy and crude protein; enzyme status represented a within-unit repeated measures factor. Each experimental period consisted of a 10-day acclimation to dietary treatments, a 2-day fecal collection, a 7-day acclimation to exogenous enzyme administration, and another 2-day fecal collection during which enzyme was administered. Horses were introduced to successive diet treatments for 7 days between experimental periods, and feed allowance was increased as needed to ensure that horses maintained a body condition score of approximately 5. Diets were fed twice daily at 12-hour intervals, and water and plain salt were available for ad libitum consumption. Enzyme product was administered orally as a molasses slurry via syringe at time of feeding. Fecal samples were collected with rectal grab at 8-hour intervals advanced by 4 hours on the second day of collection, and apparent digestibility coefficients for plant cell wall constituents were calculated with reference to acid-insoluble ash concentrations in feed and fecal dry matter (DM). Digestibility of DM was lower (P < .05) for the alfalfa-based diet than for other diets (62.0% versus 69.4%), and a diet × enzyme interaction was observed such that digestibility of DM in the alfalfa-based diet was decreased (P < .10) and in the sweet feed-based diet was increased (P < .10) when enzyme was administered. Digestibility of neutral detergent fiber tended to be lower (P < .12) for the alfalfa-based diet than for other diets (53.9% versus 60.0%), and a diet × enzyme interaction was observed such that digestibility of neutral detergent fiber in the alfalfa-based diet was decreased (P < .10) and in the oats-based and sweet feed-based diets was increased (P < .10) when enzyme was administered. Digestibility coefficients for acid detergent fiber (ADF) and hemicellulose were not different among diets, but diet × enzyme interactions were observed such that digestibility of ADF in the alfalfa-based diet was decreased (P < .10) and digestibility of both ADF and hemicellulose in the oats-based and sweet feed-based diets were increased (P < .10) when enzyme was administered. Results suggest that exogenous fibrolytic enzymes have the potential to improve digestibility of plant cell wall constituents in typical graminaceous feeds fed to horses, thereby reducing the amount of feed that would be needed to meet digestible energy requirements. Further research is needed to elucidate why exogenous fibrolytic enzymes were ineffective when administered with the alfalfa-based diet and actually resulted in decreased digestibility of DM and plant cell wall constituents.