The nutritive value of rice straw (RS) and Pleurotus ostreatus spent rice straw (SRS) was studied by analyzing its proximate composition, fiber fractions, in vitro digestibility, amino acids content and scanning electron microscopy (SEM). The possibility of replacing berseem clover (BC; Trifolium alexandrinum) with SRS at different levels also was studied. Results showed higher protein content for SRS compared to RS (3.4 to 11.7%) while, DM, OM, NFE, CF, NDF, ADF, ADL, hemicellulose and cellulose were less for SRS than for RS. Highest concentration of amino acids (mg/100 g) was in SRS compared to RS. The SEM showed an extensive damage of SRS when compared to RS. Data also showed that SRS had higher in vitro dry matter disappearance (DMD) and in vitro organic matter disappearance (OMD) compared to RS. Results of in vitro study also, indicated that the levels of 50 and 90% replacement had the highest values of DMD and OMD compared to the other levels. It could be concluded from this study that treatment of RS with Pleurotus ostreatus improved the potential feeding value of the resultant substrates (i.e. SRS) as feed resources for ruminants and possibility of replacing BC with SRS at high levels of up to 50 or 90% from diets
