The bio-sorbent used in the study was prepared from wood-decay fungus Fomes fomentarius. The chemical modification of bio-sorbent with ferric hexacyanoferrate(II) (Prussian blue) for cesium sorption studies was used. The surface area was calculated by adsorption of argon at 77 K, in accordance to the method of Brunauer et. al, 1938. The surface area of the 0.5 mm grains was 39 m2 g-1. In this study, the effect of pH in the range of 1.0–13.0 on cesium sorption onto modified bio-sorbent was studied. It was found, that the bio-sorbent has a high 137Cs absorbability in the wide range of pH, even in the solutions with high ionic strength. The maximum sorption capacity of the bio-sorbent was 50 mg Cs+ / g sorbent. The effect of competing ions Na+, K+, Ca2+ and Mg2+ was tested. It was found, that the tested cations had no a significant effect on the sorption of 137Cs. It was found, that the most suitable medium for the elution of 137Cs from the bio-sorbent was HNO3 with a concentration higher than 7 mol∙dm-3. The developed method for 137Cs determination by using modified bio-sorbent was applied to various types of waters delivered by Water Research Institute in Bratislava. 137Cs was measured on HPGe detector and 137Cs activity was less than the minimum detectable activity (0.004 Bq L-1) with the exception of water Sokolianky – Tornyosnemeti. The tested modified bio-sorbent is suitable for pre-concentration of 137Cs in water.