ABSTRACTControlled gas environments were maintained by a novel aeration system in solid substrate fermentations for enhanced protein recovery from pressed alfalfa residues. High oxygen pressures stimulated biomass and enzyme production by an Aspergillus species, isolated from alfalfa, which produced cellulase and pectinase activities in growth‐associated metabolism. High carbon dioxide pressures also stimulated enzyme production, but had less effect on biomass production, as estimated from the dissimilation of plant solids. Cellulase and pectinase activities were generally related to protein recoveries. Recoveries were up to 50% higher than those obtained by mechanical extraction, with maximum recoveries of nearly 70% of crude protein contents. Proteins not recovered at high cellulase and pectinase activities were believed to be in structurally bound forms not amenable to recovery by non‐proteolytic enzymes. Buffering at pH8 and autoclaving of residues prior to fermentation had little effect on protein recoveries. Controlled gas environments are seen to offer an interesting potential for optimizing industrial fermentation processes for the production of microbial enzy