A cybernetic model for microbial growth on mixed substrates, was used to simulate the anaerobic fermentation of cheese whey and multiple sugars in semisynthetic media by Kluyveromyces marxianus CBS 397. The model simulations quite successfully predicted the observed behavior in batch and during transients in continuous operation, in single-substrate systems as well as in media involving multiple substrates, and in semisynthetic and reconstituted cheese whey solutions. The results of simulations and their comparison with the experimental data are presented.