A method of supplying CO2 to photosynthetic algal cultures was developed based on mass transfer measurements of CO2 through microporous hydrophobic hollow fibres for various gas and liquid flow rates. A mathematical model was derived to describe the mass transfer. The designed hollow fibre module led to overall mass transfer coefficient values ranging from 1.26 x 10(-3) to 2.64 x 10(-3) cm s(-1). Higher efficiencies of the CO2 transmission were obtained at high liquid flow rates and low gas flow rates. The use of microporous hydrophobic hollow fibres enabled an enhancement of the carbon dioxide transfer per area of membrane surface by a factor of 10, in comparison to operation with silicone tubing. The hollow fibre module was operated in an external bypass to a 1 dm(3) microalgae culture vessel. In this system the algal growth pattern was similar to that obtained with a control culture where CO2 was bubbled. However, the dissolved oxygen concentration was always lower in the vessel in which CO2 was supplied by the module.