Gas hold-up, e, liquid velocity, Jt,, axial dispersion coefficient, Ez, and mixing time, t,,, have been measured in a concentric tube airlift bioreactor (12 × 10-3 m 3 in volume) using sea water, as a function of the superficial gas velocity, Jo,, (up to 0.21 m/s). Seven different spargers were tested. Four of them were cylindrical (pore size from 60 #m to 1 × 10-3 m) and three were porous plates (pore size from 30 to 120#m). Three different flow regimes are observed in hold-up and liquid velocity over the range of JG, covered: uniform bubbly flow at low gas velocities, heterogeneous flow at high gas velocities and a transition flow between bubbly and heterogeneous. The spargers with smaller pore sizes produce more hold-up and slow down velocity, because more gas recirculates into the downcomer. The change from uniform bubbly flow to transition flow appears because of the start of coalescence. In heterogeneous flow, where the bubble size is set by the degree of coalescence, no influence of the difference between the spargers used could be detected. Gas hold-up in the riser, ~,, could be represented, over the whole range of JGr used, by