The study estimated the proteolytic activity (against Hide Powder Azure) and haemolytic activity (against horse erythrocytes) in cell-free filtrates (CFF) from four strains of Aeromonas hydrophila growing under a range of commercially relevant modified atmospheres (2% O-2, 78% N-2, 20% CO2; 10% O-2, 80%, N-2, 10% CO2; 50% N-2, 50% CO2; 100% CO2). The examined strains exhibited significant qualitative and quantitative differences in the extent and times of onset of expression of these enzymes under aerobic and modified atmospheres. No proteolytic or haemolytic activities were detected in any Aer. hydrophila cultures grown at sub-optimal temperatures under modified atmospheres containing high concentrations of CO2 (i.e. 50% CO2 or 100% CO2). Although Aer. hydrophila can grow rapidly in modified atmospheres, the overall spoilage and pathogenic potential is grossly affected. Implications of these findings are discussed.