Solid sisal waste fractions which included composted sisal boles and sisal leaves decortication residues supplemented with cow dung manure at various rates used singly and/or in combination as substrates were investigated for cultivation of oyster mushroom (Pleurotus HK-37). The effect of the test sisal waste substrates and cow dung manure of various supplementation rates were evaluated by mushroom yield, biological efficiency and mushroom size. Pinheads occurred in all substrates within 3 to 5 weeks of transfer of bags to the cropping room. The overall best results of mushroom production were obtained in a substrate combination of 50 % sisal leaves + 50 % sisal boles (based on 450 g wet weight substrate) supplemented by 30 % cow dung manure with the mushroom yield of 184.64 g fresh mushrooms/kg moist substrate weight and percentage biological efficiency (B.E) of about 63 %. Mushroom size of 6.10 was obtained in sisal boles substrate supplemented by 20 % cow dung manure. Least yield of 26.73 g fresh mushrooms/kg moist substrate weight and lowest B.E of 8.95 % were obtained from non-supplemented substrate of sisal leaves alone. The study concluded that, supplementation using cow dung manure may play an important role on increasing the yield and productivity of Pleurotus HK-37 on solid sisal waste fractions under the conditions investigated.