Growth trial was conducted on twenty four Ossimi lambs, averaged 21.75 Kg live body weight and aged three months old.  The lambs were assigned randomly to 6 similar groups of (4 lambs each) and housed in open pens.  Lambs fed rations contained 60% of crude protein requirements according to NRC (1985) from concentrate feed mixture (CFM) plus untreated or fungal treated rice straw or corn stalks ad. libitum. The main results obtained confirmed that RS was more consumable and reflected lower levels of blood urea and creatinine and alkaline phosphates (ALP) activity. The fungus plus soybean meal treatment led to the highest final animal live body weight, growth rate, and feed consumption and the best feed conversion.  It gave also the highest concentrations of blood total protein and globulin and the lowest activity of the enzymes AST, ALT and ALP. It was the best economically. Conclusively, the biological treatment with the white rot fungi, particularly with the fungus Pleurotus ostreatus of the field wastes (roughages) can improve their economical and save use to narrow the feeding gap as well as to protect the environment from their accumulation or burning them causing environmental pollution.  So, fungal treatment of agricultural by-products can offer unconventional animal feed which is economical and environmentally friend without any negative effects on animal health.