The radiocaesium concentration factors (CF) of different fish species in 10 lakes in Russia, Belarus and Ukraine were measured between 6 and 11 yr after the Chernobyl accident. Clear inverse relations were observed between fish CF and lakewater K+ concentration. Perch (a predatory species) had CFs which were two times higher than non-predatory fish. No differences in CFs were observed between different species of non-predatory fish. An empirical model for the prediction of radiocaesium CFs in fish (Rowan & Rasmussen, 1994) was blind-tested against our measurements. The model predictions, based on measurements of K+ and suspended solids concentrations in the lakewater, were in good agreement with measured values. Our observations, however, implied a stronger effect of K+ on CF than that used in the Rowan and Rasmussen (1994) model. Further improvements in models could also be made by accounting for the effect of fish size on CF.