Controlled atmosphere (CA) storage prolonged the shelf life of mushrooms (Agaricus bisporus, [Lange] Sing.) if the O2 concentration was 9% or the CO2 concentration was 25 or 50%. Concentrations of 2 to 10% O2 stimulated pileus expansion and stipe elongation with maximal stimulation of growth occurring at 5% O2. Levels of CO2 above 5% markedly inhibited growth, even after air was substituted for the CO2 treatment. Five percent CO2 stimulated stipe elongation and suppressed pileus expansion. Protein degradation, as indicated by protease activity and the level of a-amino N in the tissue, increased during postharvest maturation of mushrooms. As in starving bacteria, it is suggested that the main physiological function of proteolysis in the postharvest maturation of mushrooms is as a source of C and N.