Aquatic invertebrates excrete pellets [1,2], which consist of products of incomplete digestion and pseudofeces. Different taxons are characterized by the following values of food assimilability (in percents): Rotatoria, 48‐80; Bryozoa, 41.6; Gastropoda, 42‐82; Bivalvia, 40‐47; Cladocera, 50.5‐85.5; Copepoda, 30‐ 88; Mysidacea, 84.2‐95; Isopoda, 68; Amphipoda, 5.5‐ 98; Decapoda, 38.7‐96.1; larvae of Odonata, 20‐97.2; Ephemeroptera, 41‐72; Plecoptera, 9‐73; Trichoptera, 5‐51; and Diptera, 1‐31.4 [2]. When settling to the bottom of water bodies by gravity, pellets contribute to vertical fluxes of chemical elements through the ecosystem level of the biospheric biogeochemical cycles or into “biogenic migration of atoms in the biosphere” [3]. The purpose of this study was to estimate the capacity of mollusk pellets for contributing into the vertical transfer of chemical elements through an aquatic ecosystem, using Limnaea stagnalis (L.) and bivalves (Unionidae) as examples. The mollusks L. stagnalis were collected in June in a pond in the floodplain of the upper Moskva River and were kept afterwards as described in [4]. Bivalve mollusks (Unionidae) collected from the partly silted sand bottom of the Moskva River upstream of the town of Zelinograd represented a sample from a natural benthic community dominated by Unio tumidus and U. pictorum (63.21 and 27.36%, respectively, of the overall number of specimens in the sample). The proportions of Crassiana crassa and Anodonta cygnea were lower (7.55 and 1.89%, respectively). The bivalves sampled from the natural ecosystem (the total biomass was 3302 g wet weight, including the shells; the average weight of one mollusk was 21.9 g), where they filtered natural seston, were incubated for 24 h in a wide flask containing settled tap water to obtain the pellet precipitate, which was afterwards resuspended in a 300-ml glass cylinder. The sand fraction, which precipitated within 15 s (fraction 1), was separated from the remaining suspension of the pellet material per se. The latter was transferred into another glass cylinder. From this suspension, pellets precipitated within 3 h (fraction 2); this precipitate was separated by means of decanting the supernatant. The dry weight of fraction 2 was 1434 mg. To determine the carbon content in the vegetative material and in pellets, they were oxidized by 10% K 2 Cr 2 O 7 in the presence of a mixture of concentrated H 2 SO 4 and H 3 PO 4 [5]; the CO 2 formed was trapped by 0.5 M NaOH, and the remaining alkaline was titered with hydrochloric acid. The photometric method was also used; the optic density of the solution was measured at a wavelength of 590 nm after oxidation of the material with bichromate. The amount of organic nitrogen was measured using a Kjeltec Auto 1030 Analyzer (Tecator, Sweden) by the Kjeldahl method after mineralization with a mixture of H 2 SO 4 and H 2 O 2 . Phospho