The deposition from Chernobyl in spring 1986 was most unevenly distributed in Finland and elevated the 137Cs contents of fishes in Finnish lakes significantly. High activity concentrations of 137Cs still occur in fish from certain lakes in the areas of the highest deposition. Activity concentrations of 137Cs in perch varied from 20 to 7 800 and those in lake water from 4 to 330 Bq/m3 in 1998 and 2002. Concentration factors (Bq/kg in perch / Bq/kg in lake water) ranged from 1 300 to 30 000 in the lakes studied. Ecological halftimes of 137Cs in perch in certain lakes, monitored regularly since 1986, and representing different types in terms of environmental transfer, were estimated as examples. The longest halftimes of 137Cs in perch were approximately 8 years and the shortest approximately 3 years determined for the time period of 1988-2002. Except for estimation of radiation doses to people eating fish, data on changes of activity concentrations in various parts of the aquatic system are also needed when estimating radiation doses to fish and other biota. Slow recovery of the habitat and feed from 137Cs results in higher exposure of biota than in case of rapid activity decrease.