With a bioaccumulation system using Rhodococcus erythropolis CS98 for recovery of cesium-137, we found that Cs-137 accumulated when a carbon source was added for energy supply. With the addition of ammonium acetate as the carbon source, almost all the Cs-137 from deionized water was recovered using a cell suspension of 1 g/l with incubation for 24 h. Cell damage by radioactivity was not detected during the 24 h period. Cs-137 recovery from river water samples was lower than that from deionized water, especially from river water with a very high potassium concentration (the lower reaches of the Sakura River : potassium concentration = 4.3 mg/l). When 3.9 mg/l of potassium was added to a deionized water sample, Cs-137 recovery decreased to 35% of that without potassium addition, suggesting that the potassium concentration is a critical factor for Cs-137 recovery. We conclude that a bioaccumulation system with a semipermeable membrane tube, such as is described in this paper, is feasible for the recovery of radioactive cesium from fresh waters.