AbstractTwo lignolytic mutants (POM1 - U.V. irradiated and POM2 – X ray irradiated) of P. ostreatus wild type (POW) were developed and used in new feed formulations for ruminants. Paddy straw (10 kg) amended with coconut cake, glyricidia leaves, urea (2%), and rice bran (5%) along with mutant forms of P. ostreatus substantially increased the reducing sugars, crude protein, and In Vitro Dry Matter Digestibility (IVDMD), while reducing the lignin contents. Maximum amounts of reducing sugars (555 mg/100 ml) were reported in ration 1 with strain POW in 10 days of incubation period, while a minimum was recorded in POM1 (380 mg / 100 ml) in ration 1 in 30 days of incubation. U.V. irradiated mutant was responsible for accumulation of high crude protein (CP : 42.1 mg / 100 g) in ration 1 after 30 days of incubation. The percent lignin loss was more by mutant forms, and this loss was increased with increased incubation. As a consequence, IVDMD% has been gradually improved and maximized with feed formulations 3 (49.71%) and 1 (47.07%).