This research was performed to evaluate the effects of increasing doses of exogenous fibrolytic enzymes (EFE), namely at 0 (control), 1 (low), 2 (medium) and 4 (high) μL/g dry matter (DM), on the ruminal fermentation of low-quality agro-industrial by-products, grape pomace, almond hull and pomegranate peel. For pomegranate peel, the addition of EFE increased gas production linearly from the immediately soluble fraction, and from the insoluble fraction, the potential of gas production (significant linear effect), organic matter digestibility and metabolizable energy. The highest doses were the most effective. As the doses increased, the effectiveness increased accordingly. Unlike pomegranate peel, EFE had a quadratic effect on the indices of almond hull, namely a significant response on gas production from the immediately soluble fraction, the insoluble fraction, potential of gas production, organic matter digestibility and metabolizable energy. The lowest dose was the most effective while the highest dose had inhibitory effects for gas indices, organic matter digestibility and metabolizable energy. On the other hand, this supplementation had no effect on the ruminal fermentation of grape pomace. This study indicates that the response to this feed additive was influenced by the dose, the structure of the substrate, and the interaction between dose and the substrate. Exogenous fibrolytic enzymes could enhance the use of almond hull and pomegranate peel in ruminant nutrition, and reduce their negative environmental impact.