In their natural environment microorganisms encounter changes in substrate availability, involving either nutrient concentrations or nutrient types. They have to adapt to the new conditions in order to survive. We present a model for slow microbial adaptation, involving the synthesis of new enzymes, in response to changes in the availability of substitutable substrates. The model is based on reciprocal (or mutual) inhibition of expression of both the substrate-specific carriers and the associated assimilatory machinery. The inhibition kinetics is derived from the kinetics of synthesizing units. An interesting property of the adaptation model is that the presence of a single limiting resource results in a constant maximum specific substrate consumption rate for fully adapted microorganisms. Because the maximum specific consumption rate is not a function of substrate concentration, for growth on one substrate, the Monod and Pirt models for instance are still valid. Other adaptation models known to us do not fulfil this property. The simplest version of our model describes adaptation during diauxic growth, using only one preference parameter and one initial condition. The applicability of the model is exemplified by fitting it to published data from diauxic growth experiments. © 2003 Elsevier Ltd. All rights reserved.