The accident of the Fukushima Dai-ichi Nuclear Power Plant in March, 2011, released large amounts of radiocesium into the atmosphere, and contaminated the environment of Gunma Prefecture in eastern Japan. In particular, 640 Bq kg(-1)-wet of radiocesium concentration was found in wakasagi (Hypomesus nipponensis) in Lake Onuma on Mt. Akagi in August, 2011. Thus, to elucidate the body-size effect in weight-dependent and dynamics of radiocesium in the ecosystem of Lake Onuma, we determined the age of wakasagi, the body-weight class of the radiocesium concentration in wakasagi and the effective ecological half-life ( Tee) of radiocesium in wakasagi 0+ from 2012 to 2016. The body-size effect was found for the Cs-137 concentration of wakasagi fished from 2012 to 2015, i.e., the concentration of Cs-137 increased with the increase in its body weight. On the other hand, no body size effect was found in 2016. This result may be caused by the following two factors: the Cs-137 concentration of the lake water reached a steady state after May 2014; wakasagi is a small plankton-feeding fish, while it is known that larger piscivorous fishes show the strong body size effect. T-eef of the Cs-137 concentration in wakasagi 0+ consists of two components, fast-and slow-term ones, and the decay rate of the Cs-137 concentration in wakasagi 0+ was greatly reduced. As stated above, the radiocesium contamination in Lake Onuma has still been lasting; we are thus continuing our monitoring studies.