Saccharomyces cerevisiae cells express three defined potassium-specific transport systems en-coded by TRK1 , TRK2 and TOK1 . To gain a more complete understanding of the physiological function of these transport proteins, we have constructed a set of isogenic yeast strains carrying all combinations of trk1 Δ, trk2 Δ and tok1 Δ null mutations. The in vivo K + transport characteristics of each strain have been documented using growth-based assays, and the in vitro biochemical and electrophysiological properties associated with K + transport have been determined. As has been reported previously, Trk1p and Trk2p facilitate high-affinity potassium uptake and appear to be functionally redundant under a wide range of environmental conditions. In the absence of TRK1 and TRK2 , strains lack the ability specifically to take up K + , and trk1 Δ trk2 Δ double mutant cells depend upon poorly understood non-specific cation uptake mechanisms for growth. Under conditions that impair the activity of the non-specific uptake system, termed NSC1, we have found that the presence of functional Tok1p renders cells sensitive to Cs + . Based on this finding, we have established a growth-based assay that monitors the in vivo activity of Tok1p.