Propionibacterium acnes, Propionibacterium avidum and Propionibacterium granulosum were grown in continuous culture at 0-100% air saturation using a semi-synthetic medium. Maximum specific growth rate, biomass concentration and extracellular lipase, hyaluronate lyase and phosphatase activities were determined. All three species were capable of growth at 100% air saturation but at reduced growth rates. The presence of oxygen altered the production of extra-cellular enzymes. Propionibacterium avidum was the best adapted for growth in aerobic environments.