TRK1, the gene encoding the high affinity K+ transporter in Saccharomyces cerevisiae, is nonessential due to the existence of a functionally independent low affinity transporter. To identify the gene(s) encoding the low affinity K+ transporter, we screened trk1 delta cells for mutants (Kla-) that require higher concentrations of K+ in the medium to support growth. trk1 delta trk2 mutants require up to tenfold higher concentrations of K+ to exhibit normal growth compared to trk1 delta TRK2 cells. K+ and 86Rb+ transport assays demonstrate that the mutant phenotype is due to defective K+ transport (uptake). Each of 38 independent mutants contains a mutation in the same gene, TRK2. Cells deficient for both high and low affinity K+ transport (trk1 delta trk2) exhibit hypersensitivity to low extracellular pH that can be suppressed by high concentrations of K+ but not Na+. TRK1 completely suppresses both the K+ transport defect and low pH hypersensitivity of trk2 cells, suggesting that TRK1 and TRK2 are functionally independent.