Cesium (Cs) is an alkali metal with radioactive isotopes such as137Cs and134Cs.137Cs, a product of uranium fission, has garnered attention as a radioactive contaminant. Radioactive contamination remediation using microorganisms has been the focus of numerous studies. We investigated the mechanism underlying Cs+resistance inMicrobacteriumsp. TS-1 and other representative microorganisms, includingBacillus subtilis. The addition of Mg2+effectively improved the Cs+resistance of these microorganisms. When exposed to high concentrations of Cs+, the ribosomes of Cs+-sensitive mutants of TS-1 collapsed. Growth inhibition ofB. subtilisin a high-concentration Cs+environment was because of a drastic decrease in the intracellular potassium ion concentration and not the destabilization of the ribosomal complex. This is the first study demonstrating that the toxic effect of Cs+on bacterial cells differs based on the presence of a Cs+efflux mechanism. These results will aid in utilizing high-concentration Cs+-resistant microorganisms for radioactive contamination remediation in the future.