[en] The radionuclide uptake in fish tissues obeys general regularities. Species, fish age, diet character, ratio of quantities of water and food, temperature, a season, degree of water mineralization as well as chemical properties of an element to which the radionuclide belongs, in particular, its permeability through semipenetrable partitions influence the level of uptake. Radionuclides are taken up into the fish bodies from water by the mills, skin tissues, as well as through food. Experimental studies following a single contamination of water reservoirs with nuclear fission products have shown that the time for the maximum uptake of different nuclides in the fish tissues is rather different and varies from 3 to 35 days. The maximum uptake of ⁹⁰Sr, whose concentration decreases slowly in water because of its weak sorption by soil, is attained later than that of ⁹¹Y and ⁹⁵Zr, whose concentration in water decreases quickly due to their sorption by suspended-in-water soil particles and due to sedimentation of the latter