Immobilized cells of Acetobacter sp. were used as a model system of aerobic cells to study the influence of gel particle density in fermentation carried out in fluidized-bed bioreactor. Particles of variable density were prepared by adding different amounts of alpha-alumina to the gel matrix. The effect of the density of the particles was analyzed through the variation of their terminal velocity. The behavior of continuous oxidation of ethanol to acetic acid was characterized in terms of productivity of acetic acid and gas-liquid volumetric oxygen transfer coefficient, as a function of dilution rate. An empirical correlation among the gas-liquid volumetric oxygen transfer coefficient, volumetric particle concentration (epsilon(S/L)), and particle Reynolds number (Re-p) was proposed in this work.