Mixing behavior of three geometrically equivalent 0.06 m(3) bubble column and airlift photobioreactors was compared for use in microalgal culture. Mixing was characterized in terms of the axial dispersion coefficient, mixing time, circulation time and Bodenstein number. Measurements were reported in tap water and sea water, as used in growing microalgal biomass. Compared with the two airlift reactors (a split-cylinder and a draft-tube sparged device, both of aspect ratio 9.1), the bubble column (aspect ratio = 9.0 had a shorter mixing time at any aeration rate (superficial aeration velocity of up to 0.05 m s(-1) in all reactors). Values of the various mixing parameters in the two fluids were quite comparable at identical aeration rates in any specific geometry of the reactors. Existing correlations of mixing times satisfactorily correlated the data in the bubble column, but not in the airlift reactors. The following correlations were established for the airlift devices: Bo(LG) = beta(Fr-1/3)(lambda) Bo = k( t(m) / t(c) ) where Bo is the liquid-phase Bodenstein number, Fr is the Froude number, Bo(LG) is the Bodenstein number based on the superficial velocity of the gas(,) t(m) is the mixing time, t(c) is the circulation time, and k, beta and gamma are constants that depend on the geometry of the reactor and the fluid used.