The growth of micro-organisms on disaccharides, like lactose, is highly dependant on the enzymic degradation rate of these disaccharides. After degradation of lactose, growth essentially takes place on a mixture of glucose and galactose present in the fermentation broth. The catabolic repression of glucose on the synthesis of P-galactosidase and the uptake of galactose controls growth. This is demonstrated by batch growth experiments on the growth of Lactobacillus bulgaricus on lactose. An optimal model is developed to simulate growth on lactose and on mixtures of glucose, lactose and galactose. The model is able to predict the effect of a glucose pulse during growth on lactose.