The composition and structure of the ichthyofauna of the rivers of the Romincka Forest (Pregola River basin, northeastern Poland) was investigated. Twenty-two fish species were confirmed to occur, four of which are protected: bullhead, Cottus gobio L.; spined loach, Cobitis taenia L.; bitterling, Rhodeus sericeus (Pall.); stone loach, Barbatula barbatula (L.). Species richness was the highest in the Bludzia River, in which 17 fish species belonging to 6 ecological reproductive guilds were confirmed. The ichthyofauna of the other rivers in the forest generally comprised 6-10 fish species belonging to 4-5 ecological reproductive guilds. The number of fish occurring at sampling sites decreased with increased river gradient, while the number of rheophilic species increased.