A large number of biological processes are developed with fluidized beds as reactor. This technology allows a higher biomass retention and thus a better efficiency. Some anaerobic processes such as ethanol fermentation, anaerobic digestion or denitrification are characterized by an important gas production. Nevertheless, gas production is seldom taken into account in the description of liquid mixing. In this paper, we present an experimental study of mixing and phase hold-ups in gas producing fluidized beds. The properties of gas producing and classical gas injected three-phase fluidized beds are compared. For a same gas flow rate, gas hold-ups in gas generated systems appears to be much higher than in gas injected reactors. This is attributed to the gas generation mechanism, which provides smaller bubbles at lower velocities. The degree of axial mixing is also different when gas in generated in situ than when it is injected. The dispersed plug-flow model can be reasonably used to describe the phase mixing in the reactor. The nature of bubble population appeared to be the main origin of differences between the two modes of gas introduction.