The aim of the study was to examine the impact of the European beaver, Castor fiber L., on the ichthyofauna of Negrylów Stream. Three study sites were designated in segments of the stream with running waters and two in beaver ponds. The waters at all the sites were characteristic of naturally polluted mountain streams. The occurrence of brown trout, Salmo trutta trutta m. fario L.; Siberian sculpin, Cottus poecilopus Heckel; common minnow, Phoxinus phoxinus (L.); and stone loach, Barbatula barbatula (L.), was confirmed. The highest density and abundance of brown trout was noted in the ponds, where the mean lengths and weights of brown trout were also the highest noted in the current study. Large trout occurred only in the ponds. In the shallow, running segments of the stream mostly brown trout fry were caught. Differences among fish assemblages in the segments of the stream and the ponds were statistically significant. After the introduction of the beavers, the state of the ichthyofauna in Negrylów Stream, which had suffered substantial degradation from forestry works, improved markedly. In comparison to analagous segments of other streams in the Bieszczady Mountains, fish density and biomass here were very high.