Thick shelled river mussel Unio crassus (included in IUCN Red Data List) is a benthic, filter-feeding animal very susceptible to any changes of water chemistry. In this paper we report on the new sites of this species in the Małopolska region - an area of strong anthropogenic pressure. Physical and chemical parameters have been compared between 8 rivers inhabited and not inhabited by the mussel. Results indicate that despite economic development of the region, it retains clear rivers; however, subtle differences in water chemistry can influence the occurrence of this endangered species.