A bulk treatment of substrate was developed for the cultivation of Shiitake mushrooms (Lentinus edodes) on a 1:1 mixture of cotton and wheat straws (CWS). The treatment is based on bulk pasteurization of CWS and the cultivation of mushrooms on 20-kg units of it. The advantages of this treatment lie in the reduction of energy and labor costs and the utilization of abundantly available wastes for mushroom cultivation.