The physiological effects of caesium (Cs) on living cells are poorly understood. Here, we examined the physiological role of Cs+ on the activity of the potassium transporters in E. coli. In the absence of potassium (K+), Kup-mediated Cs+ uptake partially supported cell growth, however, at a much lower rate than with sufficient K+. In K+-limited medium (0.1 mM), the presence of Cs+ (up to 25 mM) in the medium enhanced growth as much as control medium containing 1 mM K+. This effect depended on the maintenance of basal levels of intracellular K+ by other K+ uptake transporters. Higher amounts of K+ (1 mM) in the medium eliminated the positive effect of Cs+ on growth, and revealed the inhibitory effect of high Cs+ on the growth of wild-type E. coli. Cells lacking Kdp, TrkG and TrkH but expressing Kup grew less well when Cs+ was increased in the medium. A kdp mutant contained an increased ratio of Cs+/K+ in the presence of high Cs+ in the medium and consequently was strongly inhibited in growth. Taken together, under excess Cs+ conditions Kup-mediated Cs+ influx sustains cell growth, which is supported by intracellular K+ supplied by Kdp.