A simple variation of Monod kinetics is presented which describes extended lag phases observed in microbial cultivation. This variation of Monod kinetics can be expressed in a compact exponential form and is used to explain the adaptation of mixed cultures to adverse environmental conditions. The performance of the Monod model and the proposed exponential model in predicting the behavior of a mixed culture exposed to high nitrate concentrations are compared. A parameter for nitrate degradation is defined which is correlated to changes in the maximum specific growth rate as the culture adapts to increasing concentrations of nitrate.