The authors compared the differences in tolerance to oxygen of the anaerobic periodontopathic bacteria Fusobacterium nucleatum and Porphyromonas gingivalis, and explored the possibility that F. nucleatum might be able to support the growth of P. gingivalis in aerated and CO2-depleted environments. Both micro-organisms were grown as monocultures and in co-culture in the presence and absence of CO2 and under different aerated conditions using a continuous culture system. At steady state, viable counts were performed and the activities of the enzymes superoxide dismutase and NADH oxidase/peroxidase were assayed in P. gingivalis. In co-culture, F. nucleatum was able to support the growth of P. gingivalis in aerated and CO2-depleted environments in which P. gingivalis, as a monoculture, was not able to survive. F. nucleatum not only appeared to have a much higher tolerance to oxygen than P. gingivalis, but a significant increase in its numbers occurred under moderately oxygenated conditions. F. nucleatum might have an additional indirect role in dental plaque maturation, contributing to the reducing conditions necessary for the survival of P. gingivalis and possibly other anaerobes less tolerant to oxygen. Additionally, F. nucleatum is able to generate a capnophilic environment essential for the growth of P. gingivalis.