A potassium transport mutant of Escherichia coli is described which is deficient in the intake of potassium. The phenotype of this mutant is characterized by (i) failure to grow in K(+)-deficient medium, (ii) failure to accumulate K(+) in K(+)-deficient medium, (iii) a steady-state intracellular K(+) that varies sigmoidally with the medium K(+) concentration, (iv) a signoidally shaped rate-concentration curve and a curved reciprocal plot for net K(+) uptake kinetics, and (v) a low steady-state flux of potassium associated with a reduced influx rate constant. The data are discussed in terms of the present day models of cation transport. These models have led to four possible explanations of the mutant's phenotype: (i) a selectivity reversal such that intracellular cation binding sites bind another cation instead of K(+); (ii) a structural alteration of cation binding cell proteins so that K(+) is bound by "cooperative binding" (sigmoid isotherm) instead of by simple adsorption (hyperbolic isotherm); (iii) conversion of an enzyme in intermediate metabolism that rate-limits K(+) uptake to an allosteric protein; (iv) conversion of the "carrier protein" for K(+) to an allosteric protein.