We developed and applied a conductometric method for the quantitative Online measurement of the carbon dioxide (CO2) production during batch cultivations of Candida utilis on a 100-mu L scale. The applied method for the CO2 measurement consisted of absorption of the produced CO2 from the exhaust gas of the microbioreactor in an alkali solution, of which the conductivity was measured on-line. The measured conductivity change of the alkali solution showed a linear relation with the total amount of CO2 absorbed. After calibration of the CO2 measurement system, it was connected to a well of a 96-well microtiter plate. The mixing in the well was achieved by a magnetic stirrer. Using online measurement of the CO2 production during the cultivation, we show reproducible exponential batch growth of C. utilis on a 100-mu L scale. The CO2 production measurements obtained from the microcultivation were compared with the CO2 production measurement in a 4-L bioreactor equipped with a conventional off-gas analyzer. The measurements showed that on-line measurement of the CO2 production rate in microbioreactors can provide essential data for quantitative physiological studies and provide better understanding of microscale cultivations. (C) 2009 American Institute of Chemical Engineers Biotechnol. Prog., 25: 892-897, 2009