Fluidized-bed bioreactors (FBRs) with immobilized cells have been used successfully at Oak Ridge National Laboratory and at the Autonomous University of Barcelona for several applications, including high-productivity yields of ethanol. During the development of these processes, experimental results have revealed differences between three-phase fluidized biocatalyst systems and those systems typically used for hydrodynamic studies. These differences appear to be related to the physical properties (density, viscosity, surface tension) of the phases and, in particular, the low density of the solid particles. Parameters were calculated from these physical properties following the approach of Baker for two-phase flow. A map of these parameters was proposed previously and separated regions of satisfactory and unsatisfactory fluidization of a three-phase immobilized-yeast FBR fluidized with air. This same map was used for an FBR with immobilized Zymomonas mobilis that was liquid fluidized. The stability regions on the map corresponded to the two different experimental operating systems. This indicates the possible general applicability of this approach to biological FBRs. © 1994 Springer-Verlag.