Laboratorial evaluation was carried out for the effect of biological treatments with Pleurotus ostreatus, Trichodermal reesei and Chaetanium cellulyticum of rice and peas straws and corn stalks, with or without different additives on chemical composition, cell wall constituents (CWC), gross energy and in situ dry and organic matter disappearance. The obtained results could be summarized in the following:      ·  Peas straw was the best roughage among the tested straws and stalks; it gave the highest contents of crude protein, gross energy and cellulose.  It reflected also the highest in situ disappearance of either dry matter or organic matter.      ·  Pleurotus ostreatus was the best fungus among the tested fungi in increasing the treated roughage contents of organic matter, crude protein, NFE and gross energy.  It was the best also in elevating dry matter and organic matter disappearance in situ.  It led to the lowest CF and most of CWC.      ·  Fungus + soybean meal was the best treatment that led to the highest CP content and in situ disappearance of DM and OM as well as to the lowest CWC.       Conclusively, the biological treatment with the white rot fungi, particularly with the fungus Pleurotus ostreatus of the field wastes (roughages) can improve their chemical composition and nutritive value.