In large-scale fermentors, non-ideal mixing leads to the development of heterogeneous cell populations. This cell-to-cell variability may explain the differences in e. g. yields for large-and lab-scale cultivations. In this work the anaerobic growth of Saccharomyces cerevisiae in a continuously run microbioreactor is simulated. A multi-scale model consisting of the coupling of a population balance model, a kinetic model and a flow model was developed in order to predict simultaneously local concentrations of substrate (glucose), product (ethanol) and biomass, as well as the local cell size distributions.