| Summary: | This study investigated the content of <sup>137</sup>Cs (a long-lived radioactive isotope of caesium) in various parts of <i>Pinus sylvestris</i> L. (Scotch pine) and <i>Dicranum polysetum</i> Sw. (rugose fork-moss) at three different sites within the exclusion zone of the Chornobyl nuclear power plant over two years. The Leliv site is located within the 10 km zone, while the Paryshiv and Dytiatky sampling sites are within the 30 km zone. Samples of different <i>P. sylvestris</i> organs were collected, including 1- and 2-year-old branches and needles and wood and outer bark, and the entire <i>D. polysetum</i>. Sampling was conducted every two weeks throughout the year during 2014 and 2015. The specific activity levels of <sup>137</sup>Cs in the samples were measured using gamma spectrometry with a CANBERRA gamma spectrometer unit and a coaxial high-purity HPGe semiconductor detector. The study found that at the Leliv and Paryshiv sites, the highest content of <sup>137</sup>Cs in living organs of <i>P. sylvestris</i> was found in the wood. At the Dytiatky site, the needles and branches of the first and second years had anomalously high concentrations of radiocaesium (<sup>137</sup>Cs). This could be due to a thin layer of forest litter (1.5 cm) at that site. The study also found significant changes in the specific activity levels of <sup>137</sup>Cs in living pine organs throughout the year. The highest concentration was observed in pine branches and needles in summer, and the maximum values in wood were observed in winter. The study suggests that a constant circulation of <sup>137</sup>Cs in the soil–plant system can cause seasonal changes in the content of <sup>137</sup>Cs in living pine organs. Symbiotic mycorrhizal fungi can play an important role in the circulation of radiocaesium in forest ecosystems. The outer bark of <i>P. sylvestris</i> did not show any seasonal changes in the content of <sup>137</sup>Cs. It may not be involved in radiocaesium redistribution inside the plant but can serve as a long-term source of this radionuclide entering the forest litter. The study found no seasonal changes in the accumulation of <sup>137</sup>Cs by <i>D. polysetum</i>, which might be due to the physiological characteristics of this plant species. Based on the analysis of the conducted studies, the recommendation is to consider the seasonal changes in the content of <sup>137</sup>Cs during monitoring activities and when using Scots pine in areas potentially contaminated with this radionuclide.
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