In the present paper, gas dispersion in a laboratory scale (5 L) stirred bioreactor is modelled using a commercial computational fluid dynamics (CFD) code FLUENT 6.2 (Fluent Inc., USA). A population balance model (PBE) is implemented in order to account for the combined effect of bubble breakup and coalescence in the tank. The impeller is explicitly described in three dimensions using the two-phase and Multiple Reference Frame (MRF) Model. Dispersed gas and bubbles dynamics in the turbulent flow are modelled using an Eulerian-Eulerian approach with dispersed k-epsilon turbulent model and modified standard drag coefficient for the momentum exchange. Parallel computing is used to make efficient use of the computational power in order to predict spatial distribution of gas hold-up, Sauter mean bubble diameter, gas-liquid mass transfer coefficient and flow structure. The numerical results from different distribution of classes are compared with experimental data and good agreement is achieved. (C) 2007 Elsevier Ltd. All rights reserved.