Soybean hulls (SH) have been successfully fed to ruminant animals as an economical substitute for hay. This feedstuff is a source of highly digestible fiber and does not contain starch. The purpose of this trial was to evaluate SH as a replacement fiber in horse diets. Four cecally cannulated Quarter Horse geldings, aged 6 to 10 years and averaging 502 kg, were used in a 4x4 Latin Square design with 21-day periods. Diets consisted of alfalfa-bromegrass hay (14.4% CP, 58.1% NDF, 39.1% ADF) with the addition of either 0, 25, 50, or 75% unpelleted SH (13.1% CP, 60.6% NDF, 43.7% ADF). Diets were offered at 2% of bodyweight (as fed) daily and body weights were measured weekly. Cecal samples (90-min post-feeding) and total fecal collections (3 d) were taken at the end of each treatment period. Fecal collection bags were emptied every 6 hours and 10% of the total amount was frozen for later analysis. Total cecal VFA production increased linearly from 70 mM to 109 mM as proportions of SH in diets increased (P = 0.02). Proportions of propionate increased linearly (P < 0.01) and cubically (P = 0.03) with means of 15.7, 18.0, 16.6, and 21.9 moles per 100 moles total VFA for the 0, 25, 50, and 75% SH diets respectively. Proportions of butyrate decreased linearly (P < 0.01) from 5.3 to 3.9 moles per 100 moles total VFA. The acetate to propionate ratio decreased linearly (P = 0.02) and cubically (P = 0.03) with means of 4.9, 4.2, 4.9, and 3.3. Apparent digestibility of DM, OM, NDF, ADF, cellulose, and hemicellulose did not differ (P > 0.24) with treatment. Apparent digestibility of N decreased linearly (P < 0.01) as concentrations of SH increased in the diet, most likely due to increased cecal fermentation and microbial biomass production. Cecal pH decreased linearly (P = 0.01) from 7.00 to 6.45 as the level of SH increased, but there was no change (P linear = 0.68) for cecal ammonia (mean concentration of 3.85 mM). Soybean hulls appear to stimulate cecal fermentation and to be a suitable replacement for hay in equine diets. This may be an especially important finding for owners of geriatric horses that often have difficulty consuming roughages.