An alternative interpretation of the growth rate-substrate concentration dependence is presented. This is based on the assumption that the main factors affecting growth rate are transfer of substrate from the medium and the maximum growth velocity, which is that observed when no substrate limitations occur. This approach allows the approximate prediction of one of the two kinetic constants required, and may be of great use, especially for continuous cultures. It is the fi rst attempt to provide a phenomenological explanation for the large variations observed in the values of the Monod constant, K-S, reported in the literature.