The effect of oxygen on the in vitro propagation of Propionibacterium acnes was investigated under defined culture conditions. This micro-organism is the predominant bacterial resident within the pilosebaceous follicles of sebum-rich areas of human skin. The organism was grown in continuous culture in defined synthetic medium with glucose as the main carbon-energy source at various air saturation concentrations and in the presence and absence of light. Steady state continuous cultures were achieved at very low oxygen tensions in the presence of light, and at higher levels of oxygen when non-illuminated. Culture biomass yields were higher than those of anaerobic cultures. Bacterial cells were inactivated in the presence of light at high oxygen concentrations because of photosensitization reactions involving excess oxygen and microbial porphyrin species.