Microbial biomass and activity were determined in cambisol incubated under ambient and increased (up to 2.23 mmol/L) CO2 concentrations. An immediate negative response of the soil microbial community to [CO2] increase was observed during the first day with respect to microbial biomass, soil respiration and specific respiration activity (both expressed as CO2 evolution). In contrast, O-2 consumption was not affected but anabolic utilization of available substrate increased. These phenomena were observed under conditions of increased CO2 tension but without any change in O-2 concentration.