The use of citrus by-products was evaluated through the fermentation of the orange peel by means of Aspergillus niger van Tieghem strain, 1867, which was obtained by inoculation in a petri dish, at a temperature of 35°C and a pH of 4,3. The crushed orange peel was used as a substrate, partially dehydrated, spread on trays, dried in cardboard trays and without heat pretreatment, adding only components and mineral nutrients, and A. niger was cultivated at different fermentation temperatures between 8°C and 40 °C. Upon completion of the fermentation process of the pretreated orange peel, maximum percentages of transformation of mineral nitrogen to organic nitrogen of 34.5% were obtained, for a value of the thermal integral between 0°C and 320°C per day, obtaining for a kg of dry matter, 6.6% protein for 600 g of product (feed) with 18% protein. Finally, the variation of the pH of the s...