Wild-type cells of E. coli take up Mg rapidly when grown in media with high (10 mM) or with low (50 muM) concentrations of Mg. Previous work has provided evidence that a transport system for Mg in E. coli also functions for the uptake of Co. Mutants resistant to Co have now been found to possess a system for Mg uptake that is repressed by high concentrations of Mg in the growth medium, and does not function effectively for the uptake of Co. The rate of Mg uptake by such mutants increases at least 50-fold within one generation after a transfer from growth medium with high Mg to one with low Mg concentrations. We conclude that there are at least two systems for the uptake of Mg in E. coli. System I is nonrepressible and can catalyze the uptake of Co as well as Mg; it is present in wild type but not in Co-resistant mutants. System II is repressible and has little affinity for Co; it is present in both wild-type and mutant strains.