The effect of Pleurotus pulmonarius on the chemical composition of wheat straw was evaluated. Wheat straw, treated and untreated with P. pulmonarius, was obtained from a commercial facility. Ten samples plastic bags of wheat straw used previously as substrate to culture edible fungus were collected at random. The negative control group consisted of the pasteurized wheat straw untreated with P. pulmonarius. All samples were analyzed to determine dry matter, organic matter, crude protein, neutral detergent fiber, acid detergent fiber, cellulose and hemicellulose of each wheat straw. Data were analyzed by mean comparison using a t-Student test. No differences (P>0.05) between treatments were found for dry matter, crude protein and hemicellulose; however, straw treated with P. pulmonarius showed higher percentages (P<0.05) of organic matter, neutral and acid detergent fiber. It is concluded that growing P. pulmonarius in wheat straw improves the chemical composition of the straw by increasing its organic matter content and modifies the fiber structure, which increases the soluble carbohydrates content.