To test the effects of gas levels during incubation on mushroom production, we grew two strains of shiitake, Lentinula edodes, on a sawdust medium inside plastic mushroom-growing bags having different amounts of ventilation. During 77 days of incubation, oxygen and carbon dioxide levels inside the bags varied with: 1) the fungus strain, 2) the metabolic activity and stage of development of the fungus, and 3) the size of the microporous filter patch ventilating the bag. Total mushroom yield, fruitbody size, the number of mushrooms produced, and the severity of contamination during cropping were affected by filter patch size. Each strain required a different amount of ventilation in order to achieve optimum mushroom production.