The Yenisei River is polluted with transuranium elements because of operation of the Mining-and-Chemical Combine of the Ministry of Atomic Energy of the Russian Federation (Zheleznogorsk) [1,2]. One of them is 241 Am [3]. Observations in the waters of the Chernobyl zone contaminated with radioactive fallout demonstrated that aquatic plants accumulate 241 Am in their biomass in a species-specific manner [4]. Our experiments showed that Elodea ( Elodea canadensis ), a submerged aquatic plant belonging to the phytocenosis of the Yenisei River, can accumulate considerable 241 Am radioactivity [5]. The mechanism of americium accumulation by aquatic plants is still unknown. Studies on the accumulation of heavy metals and radionuclides showed substantial differences in their contents between different organs of aquatic plants [6,7]. The specific distribution among organs of macrophytes sometimes allows assumptions on the mechanisms of accumulation of some metals and their roles in plant metabolism [6]. Photoassimilating organs (leaves and shoots) account for the largest part of Elodea phytomass and are characterized by substantial ultrastructural and physiological differences [8], which may affect the intensity of 241 Am accumulation in them. The distribution of americium-241 and other transuranium elements in organs of Elodea had not been studied before.Therefore, we analyzed the characteristics of 241 Am accumulation by the leaves and stems of E. canadensis , a submerged aquatic plant.The shoots of Elodea were used for laboratory experiments. Elodea is a common component of phytocenoses throughout the Yenisei and most bodies of water in the Northern Hemisphere [6,9]. The plants were collected in the Yenisei upstream of the radioactive contamination zone in late July 2004. We used unbranched shoots with apices. The shoots were washed with tap water and exposed in glass cylinders filled with water from the Yenisei (0.7 l per cylinder) filtered through Vladipor membranes with a mesh size of 1 µ m. We used a total of 28 shoots 12-22 cm in length. The cylinders were illuminated with luminescent lamps for 14 h per day, with an illumination of 2 klx on the side surface and a temperature of 20 ± 1°ë .Americium was added to water in the form of a water solution of americium nitrate, 241 Am(NO 3 ) 3 ; then, pH was adjusted to 7 by adding NaOH, and the plants were placed into the water. During the experiment, pH 7 was maintained. The initial radioactivity of 241 Am in the water was 1340 ± 50 Bq/l.To estimate the strength of americium binding by the plant biomass, we performed chemical fractionation as described earlier [5].To measure the 241 Am radioactivity accumulated by the biomass, leaves and stems were dried until their weight became constant and then dissolved in a mixture of 30% hydrogen peroxide and 1 M nitric acid while heating. The 241 Am activity in the resultant solutions and in the water from experimental cylinders was measured by means of a γ -spectrometer with an ultrapure germanium detector (Canberra, United Stat...