Levels of oxygen and carbon dioxide in the gas environment were found to influence pigment production significantly and growth to a lesser extent in solid‐state fermentations with Monascus purpureus on rice. Maximum pigment yields were observed at 0.5 atm of oxygen partial pressure in closed pressure vessels. However, high carbon dioxide partial pressures progressively inhibited pigment production, with complete inhibition at 1.0 atm. In a closed aeration system with a packed‐bed fermentor, oxygen partial pressures ranging from 0.05 to 0.5 atm at constant carbon dioxide partial pressures of 0.02 atm gave high pigment yields with a maximum at 0.50 atm of oxygen, whereas lower carbon dioxide partial pressures at constant oxygen partial pressures of 0.21 atm gave higher pigment yields. Maximum oxygen uptake and carbon dioxide evolution rates were observed at 70–90 and 60–80 h, respectively, depending on the gas environment. Respiratory quotients were close to 1.0, except at 0.05 atm of oxygen and 0.02 atm of carbon dioxide partial pressures. Optimum conditions for pigment formation were generally not the same as those for growth.