The growth of the red microalga it Porphyridium sp was studied in three bench-scale bioreactors of 13 dm(3) volume: a bubble column, an airlift reactor and a modified airlift reactor with helical flow promoters in the top of the downcomer. Most of the experiments reported were run with a photon flux density of 250 muE m(-2) s(-1), but other illuminances were studied as well. Superficial gas velocities were in the range of 5.4 x 10(-4) to 82 x 10(-4)ms(-1) (0.033-0.5vvm). Algal growth in the airlift reactor with helical flow promoters had lower gas requirements than in the other reactor configurations. This implies lower costs in air compression and in air and CO2 requirements. It was concluded that the advantages found are related to the particular fluid dynamic characteristics of the reactor.