The Ankom fermentation system was used to investigate the effect of rumen fluid donor (sheep vs red deer) on in vitro digestibility of twelve substrates (four forages, four cereal grains and four shrubs). Dry matter and neutral-detergent fibre disappearances and in vitro true digestibility showed some significant differences related to the species of the rumen fluid donor, suggesting that although sheep could be used as a model to estimate in vitro digestibility of some conventional feeds (e.g., forages and cereal grains), important differences may be found between sheep and red deer when low-quality feeds (e.g., shrubs) are evaluated.