Regeneration of atmosphere is an essential component in a long-term manned mission in space. A compact and reliable photobioreactor (PBR) system with an efficient gas transfer module is required for this purpose. Light emitting diodes (LEDs) provide an ideal light source for a small and maintenance-free PBR. Lack of gravity in space prevents the use of sparging, one of the most efficient gas exchange processes. As an alternative gas transfer device, a hollow fiber gas exchanger was selected and examined for possible future application. An LED-based PBR with a hollow fiber external gas exchanger supported high-density algal cultures comparable to a PBR with internal sparging (>2 x 10(9) cells/ml, or over 6% v/v). The growth kinetics in both types of PBRs were found to be identical and the oxygen production rate was about the same when the effect of the dark volume in the external hollow fiber gas exchanger was taken into account. To quantitatively describe the effect of non-illuminated volume inside a hollow fiber gas exchange unit, two parameters were introduced : epsilon, which was the ratio of illuminated volume to dark volume in the entire PBR system, and phi, defined as the ratio of the specific dark respiration rate to the maximum specific oxygen production rate. The decrease in net oxygen production in a PBR with an external gas exchanger was quantitatively predicted by a simple model using these two parameters.