A shell-abounding thanatocenosis was found on the valley floor of the Osam River near Muselievo. It is composed of 42 taxa of land snails and aquatic molluscs. Six species of rheophile water snails, Lithoglyphus naticoides (C. Pfeiffer) being the dominant, are the main components of the assemblage, together constituting 57%. Land snails are represented mainly by open-country species, including the most numerous shells of Helicopsis striata (O. F. Müller). All the taxa of the thanatocenosis are known from the recent fauna of Bulgaria and adjacent countries. The percentage of particular components corresponds to thanatocenoses deposited in the lower course of the river. The shell material was displaced by the flood flow over a relatively short distance and deposited close to environments inhabited by molluscs. Differences in abundance of some species between the thanatocenosis and the malacocenosis can be explained by differences in the hillwash intensity controlled by local conditions.