Experiments were conducted to study the hydrodynamics of a gas-liquid-solid fluidized bed containing low-density particles. The density and size of the particles employed were comparable to those ordinarily encountered in fluidized-bed bioreactors for either wastewater treatment or fermentation. A dual resistivity probe and a conductivity probe were employed to study the axial holdup of gas and liquid phases and the bubble size distribution. The behavior of the axial holdup distribution for solid, gas, and liquid phases is reported. A mechanistic model, based on solids entrainment and deentrainment mechanisms by bubbles, is developed to describe the behavior of the axial solids holdup distribution. The model is shown to satisfactorily account for the experimental data.