Measurements of Cs-134 and Cs-137 in Rozites caperatus from 1991 to 2001 showed differences in the uptake pattern of Chernobyl and fallout derived radiocesium over time. The effective half-life of Chernobyl derived Cs-137 was determined to 12.4 years, whereas fallout derived Cs-137 had an effective half-life of 9.8 years. However, the trend for Chernobyl derived Cs-137 was clearly not linear. An effective average half-life of 3.1 years was determined for Chernobyl derived Cs-137 in the period from 1991 to 1994. The corresponding ecological half-life was 3.4 years. In the period from 1994 to 2001 it is possible that the effective half-life was longer than the physical, which indicates that more Chernobyl cesium becomes available than disappears through radioactive decay. The difference may be due to depth distribution of radiocesium, i.e. fallout derived cesium have penetrated deeper than Chernobyl. The changes in the half-lives of Chernobyl derived radiocesium over time together with the half-life of the older fallout derived radiocesium may be valuable in predictions of future radiocesium concentrations in mushrooms and thereby of food chain transfer to man. (C) 2004 Elsevier B.V. All rights reserved.