Several probes for measuring dissolved carbon dioxide (CO2) concentration were installed in a 68-litre fermentor and their effectiveness compared. Submerged silastic rubber tubing gave reproducible results over a wide range of operating conditions and was generally superior to all other probes evaluated.The silastic rubber probe was used to compare the partial pressure of CO2in viscous fermentation media with that in the fermentor exhaust gas. No significant difference was found.Results show that determination of the CO2partial pressure in the exhaust gas gives an excellent approximation of the partial pressure of dissolved CO2in the liquid medium, eliminating the need for measurement of CO2concentration in the broth.