Batch fermentation kinetics of Lactobacillus bulgaricus were examined in detail using the methodology of cybernetic modelling. The effect of pH and lactate ion on the activity of the enzyme B-galactosidase was simulated. Cybernetic modelling is mainly used for simulation of growth on multiple substrates. Here, it has been incorporated in a model which simulates pH effects on single-substrate batch growth. The expression of active enymze is crucial for substrate and growth. It was seen that the effect of lactate ion on the activity of the enzyme was dependant on the system pH. A semi-empirical expression was obtained for the relative amount of active enzyme present in the organism and used to model the enzyme, biomass, substrate and lactic acid concentrations for the batch fermentation. The rate of biomass formation and product formation depended on the amount of enzyme synthesised, which in turn was dependant on the pH value of the system. The model developed simulates the effect of pH and lactate concentration on the expression and degradation of the enzyme.