The present experiment aimed at increasing orange peel and sugar beet pulp protein content through solid-state fermentation by Trichoderma reesei and Trichoderma viride. In vitro digestibility and changes in the chemical composition of the fermented products were determined after seven days of fungal cultivation using gas production tests. The cultivation of T. reesei and T. viride on orange peels decreased neutral detergent soluble content [P<0.01] and increased cellulose, hemicellulose and lignin contents [P<0.01] . Changes in fiber fractions were found to be more pronounced with T. viride. The cultivation of T. reesei and T. viride on sugar beet pulp increased neutral detergent soluble content [P<0.01] and decreased cellulose and hemicellulose contents [P<0.01] . These changes were more pronounced with T. reesei. The cultivation of T. reesei or T. viride on orange peels or sugar beet pulp increased crude protein content [P<0.01] compared with the unfermented materials; however, the increase was more pronounced for orange peels fermented with T. viride when corrected for weight loss [P<0.05] . After 24 and 48 h of incubation, significant decreases in cumulative gas production [P<0.01] were observed in fermented sugar beet pulp and orange peels compared with the unfermented materials. Fungal treatment of orange peels and sugar beet pulp reduced the digestibility of in vitro organic matter, metabolizable energy and average fermentation and gas production rates [P<0.01] . The data showed that seven days of solid-state fermentation of orange peels and sugar beet pulp by T. reesei or T. viride can increase their crude protein content