Computational fluid dynamics (CFD) is becoming a powerful tool in the scale-up and design of aerated agitated bioreactors. However, most early studies consider only single-phase flow or multiphase with a constant bubble size assumption. By solving the custom-integrated population balance model (PBM), which takes into account bubble break-up and coalescense, and the multiphase flow equations in a commercial CFD code (Fluent 6) simultaneously, local gas fraction, bubble-size distribution and volumetric mass transfer rate are predicted. Model predictions for the velocity field in a 100L bioreactor is compared with LDV measurements. The model provides a reliable tool in the scale-up and design of bioreactors, where the gas transfer rate is important.