An aerobic methanotrophic-heterotrophic soil community has been characterised when growing with different partial pressures of CO2. The methanotrophic population using methane as carbon source reached 3 x 10(7) cfu ml(-1) with one of the major methanotrophs being of type II which uses the serine pathway for C assimilation. Optimal methanotrophic activity required the addition of CO2, and in the absence of CO2 no methane oxidisers grew. Partial pressures of CO2 from 1.6 to 11.6 kPa gave optimal cell growth and production of soluble organic compounds. Biomass yield, soluble organics and CO2 production were 0.36, 0.15, and 0.48 mg mg(-1) methane uptake, respectively, with CO2 at 11.6 kPa. The results presented here may have important implications for the use of methane-oxidising bacteria in bioremedial applications.