The paper describes taxonomic composition of leeches (Hirudinea) in the strongly anthropogenically altered upper reaches of the Stepnica River in Western Pomerania. The numbers of respective species of the Hirudinea were analyzed in the context of such environmental factors as: instantaneous flow, dissolved oxygen content, and calcium content. It was noted that most leech species inhabited the headstream, and factors deterrent to their occurrence included the presence of hydrogen sulfate and siltation.