This paper describes results of the application of simple metabolic model principles to aerobic growth of Klwuyveromycesfragilis during the utilisation of a semi-synthetic medium simulating dairy waste (whey). The theoretical yield coefficients (maximum, true and real) of biomass on lactose and oxygen, available electrons and ATP were determined and compared with experimental data. The experimental runs were performed in a B Brown fermenter (15 dm(3) of operating capacity). The yeast K fragilis was cultured in semi-synthetic medium containing lactose (40 g dm(-3)) as a main carbon and energy source and ammonia salts as a main nitrogen source (C/N = 8.74). The yeast growth is closely related to the concentration of oxygen in the media. The experimental yields of biomass on lactose and oxygen reached 94% and 88% of real maximum theoretical values, respectively. Calculated true biomass yield coefficients were closely correlated with the values resulting from the balance analysis of stoichiometric equations. The specific maintenance requirement (m(ATP)) was 14.7 mmole ATP(g DM h)(-1). Determination of the elemental composition of the biomass during the course of fermentation resulted in observations of the cellular C/N ratio changes. The conversion of lactose carbon to biomass and CO2 was 74.3%-81.2%. The recovery of ATP reached 79%, and the recovery of oxygen 96.2%.