Chemostat cultures in which the limiting nutrient was lactose have been used to study the relative growth rate of Escherichia coli in relation to the enzyme activity of β-galactosidase. A novel genetic procedure was employed in order to obtain amino acid substitutions within the lacZ-encoded β-galactosidase that result in differences in enzyme activity too small to be detected by ordinary mutant screens. The cryptic substitutions were obtained as spontaneous revertants of nonsense mutations within the lacZ gene, and the enzymes differing from wild type were identified by means of polyacrylamide gel electrophoresis or thermal denaturation studies. The relation between enzyme activity and growth rate of these and other mutants supports a model of intermediary metabolism in which the flux of substrate through a metabolic pathway is represented by a concave function of the activity of any enzyme in the pathway. The consequence is that small differences in enzyme activity from wild type result in even smaller changes in fitness.