Summary: Caesium accumulation by Chlorella salina, from buffer (pH 8.0) supplemented with 50 μM-CsCl and 137Cs, continued for approximately 15 h and displayed first-order kinetics, indicating a single rate-limiting transport process. Efflux of Cs+ from Cs+-loaded cells occurred in two distinct phases: a rapid initial loss, representing approximately 11% of total cellular Cs+, corresponded to release from the cell surface, whereas a second, slower, phase of efflux corresponded to loss from the cytoplasm and vacuole. Analysis of subcellular Cs+ compartmentation revealed that most Cs+ was accumulated into the vacuole of C. salina, with lesser amounts being associated with the cell surface or located in the cytoplasm. Uptake of Cs+ into the vacuole was correlated with a stoichiometric exchange for K+. However, no loss of K+ from the cell surface or cytoplasm was evident nor was Cs+ or K+ associated with insoluble intracellular components. Calculated values for the Cs+ flux across the vacuolar membrane were approximately equal to, or higher than, values for total cellular influx. Cs+ influx obeyed Michaelis–Menten kinetics over the lower range of external Cs+ concentrations examined (0.01–0.25 mM) and a single transport system with a K m ± 0.5 mM was evident. The effects of other monovalent cations on Cs+ influx implied that K+ and Rb+ were competitive, and NH+ 4 non-competitive/uncompetitive inhibitors of Cs+ uptake. The order of inhibition was Rb+ > K+ > NH+ 4. We propose that a single, relatively non-selective, rate-limiting transport system for Cs+ influx is located on the cytoplasmic membrane of C. salina, while a more permeable vacuolar membrane facilitates transport of Cs+ into the vacuole.