This study aimed at assessing the impact of four barley forms on total tract apparent digestibility of dietary fibre in horses fed a large amount of starch in the morning meal (0.27% BW). Processed barley forms had a greater pre-caecal starch digestibility than the whole form. Based on this result, we hypothesised that using barley-processing methods would limit the potential dumping of undegraded starch in the hindgut of horses and, consequently, the potential negative effect on fibre degradation in the hindgut. In a 4×4 latin square design, four mature geldings fitted with a right ventral colon-fistula were fed a meadow hay : concentrate (62 : 38; dry matter (DM) basis) diet at 1.7% BW. The concentrate was made of 80% barley distributed either as whole grain or as processed forms: 2.5 mm ground, pelleted or steam-flaked. For each period, total tract apparent digestibilities of DM, NDF and ADF were determined over 3 consecutive days by total faecal collection, whereas pH, volatile fatty acids (VFA) concentrations and cultural functional bacteria counts (total anaerobic, cellulolytic bacteria, lactic acid producers, amylolytic bacteria and lactic acid utilisers) in colonic content were evaluated on 1 day 4 h after the morning meal. Total tract apparent digestibility of DM and dietary fibre was influenced (P<0.05) by barley form. Diets including thermo-mechanically treated barley forms led to a higher (P<0.05) total tract apparent digestibility of NDF than those constituted of ground barley and also led to a greater (P<0.05) total tract apparent digestibility of ADF than those made of whole or ground barley forms. However, no significant difference was observed in colonic pH, VFA concentrations and cultural bacteria concentrations. Owing to a high starch supply in the morning meal, the concentration of the functional bacteria in the colonic content averaged 7.8 log CFU/ml, 5.9 NPM/ml, 6.9 and 7.3 CFU/ml for total anaerobic, cellulolytic, amylolytic and lactic acid-utilising bacteria, respectively. Consequently, providing horses with pelleted or steam-flaked instead of ground barley forms may limit the negative impact of starch on fibre digestibility in horses fed a high level of starch in the morning meal (0.27% BW). Moreover, the fibre-to-starch ratio fed in this experiment did not cause any digestive upset.