The effect of different gaseous phases during solid-state fermentation of wheat straw using Phanerochaete chrysosporium was studied. The cultures were allowed to grow by passive air-exchange through cotton plugs for the first 10 days and then growth was continued under different mixtures of oxygen, carbon dioxide and nitrogen for the next 20 days. P. chrysosporium improved the digestibility of wheat straw in all experiments after 30 days of growth. The loss of lignin increased with increasing carbon dioxide at a constant level of oxygen in the atmosphere. The process efficiencies with respect to change in digestibility or loss of lignin per gramme los.5 of organic matter were generally higher in gaseous treatments than under passive air-exchange. Loss of lignin correlated well (r = 0.88) with change in digestibility.