The effects of system parameters on the gas holdup in a three-phase fluidized-bed bioreactor were examined. A valve technique was used to measure the average gas holdup in the reactor. Gas holdup was found to be strongly affected by fluid superficial velocities, bead size and density, electrolyte concentration, type of gas sparger, and temperature. Quantitative and qualitative observations were made regarding the differences between conventional glass-particle systems and low-density gel-particle systems. Most notably, the degree of bubble coalescence and mode of fluidization were highly dependent on each system.