Gas hold-up, epsilon, liquid velocity, J(Lr), axial dispersion coefficient, E-z, and mixing time, t(m), have been measured in a concentric tube airlift bioreactor (12 x 10(-3) m(3) in volume) using sea water, as a function of the superficial gas velocity, J(Gr), (up to 0.21 m/s). Seven different spargers were tested. Four of them were cylindrical (pore size from 60 mu m to 1 x 10(-3) m) and three were porous plates (pore size from 30 to 120 mu m). Three different flow regimes are observed in hold-up and liquid velocity over the range of J(Gr) 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, epsilon(r), could be represented, over the whole range of J(Gr) used, by,epsilon(r), = alpha(J(Gr)/J(Lr))(beta).When the Bodenstein number for the gas-liquid mixture, Bo(LG), and the Froude number, Fr, are used to represent axial dispersion, three flow regimes appear which correspond to those observed when hold-up and liquid velocity are plotted vs J(Gr) The axial dispersion coefficient, E-z, could be represented bywhere d(e) is the equivalent diameter of the reactor. This equation suggests that E-z is dominated by bubble slip, and fits 78% of the measured data with less than 20% error. It has also been applied satisfactorily to the results obtained by other authors who used different systems and liquid phases. Mixing time depends on sparger geometry and pore size only at low gas velocities. At high gas velocities, mixing time is practically independent of sparger and gas velocity.