A continuous nutrient flow system has been developed to measure microbial activity in soil with various concentrations of added substrate. The system consists of a thin soil layer through which substrate was added continuously over periods up to 4.5 days. Substrate utilization was determined by effluent analysis. Respiration was measured manually by injecting a sample into a gas chromatograph or automatically by coupling the growth chamber to a computer-controlled gas sampling valve. This permitted respiratory CO2 to be measured by the gas chromatograph at intervals selected by the investigator. Software controlling the valve and gas chromatograph not only automated gas phase sampling, but also provided a scan of CO2 evolution and a preliminary data summary. Data for growth on glucose using a microbial population native to a California annual grassland soil demonstrated that the direct cell count and respiratory techniques for biomass estimation give comparable results. (Copyright (c) Martinus Nijhoff Publishers.)