Fungi in forest ecosystems are major contributors to accumulation and cycling of radionuclides, especially radiocaesium. However, relatively little is known about uptake and retention of 137Cs by fungal mycelia. This thesis comprises quantitative estimates of manually prepared mycelia of mainly ectomycorrhizal fungi and their possible role in the retention, turnover and accumulation of radiocaesium in contaminated forest ecosystems. The studies were conducted in two forests during 1996-1998 and 2000-2003. One was in Ovruch district, Zhytomyr region of Ukraine (51º30"N, 28º95"E), and the other at two Swedish forest sites: the first situated about 35 km northwest of Uppsala (60º05"N, 17º25"E) and the second at Hille in the vicinity of Gävle (60º85"N, 17º15"E). The 137Cs activity concentration was measured in prepared mycelia and corresponding soil layers. Various extraction procedures were used to study the retention and binding of 137Cs in Of/Oh and Ah/B horizons of forest soil. 137Cs was also extracted from the fruit bodies and mycelia of fungi. The fungal mycelium biomass was estimated and the percentage of the total inventory of 137Cs bound in mycelia in the Ukrainian and Swedish forests was calculated. The estimated fungal biomass in Ukrainian forests varied from 0.07 to 70.4 mg g-1 soil, in Swedish forests between 3.6 and 19. 4 mg g-1 soil. Between 0.5 to 50 % of the total 137Cs activity in the 0-10 cm soil profile was retained in the fungal mycelia. The 137Cs activity concentration in mycelia was thus higher than that found in soil, and 137Cs activity concentrations in the fruit bodies was higher than that in the mycelium. The survey study revealed that a major part, around 50 % of the plant-available 137Cs in forest soil, was retained in the fungal mycelium. The most probable sources of 137Cs for fungal mycelia and fruit bodies of fungi were found to be water soluble substances, humic matter, hemicellulose and cellulose
