This paper presents the first records of the occurrence of brown bullhead, Ameiurus nebulosus (Lesueur), in the Łyna River drainage basin. In February 2010, 77 brown bullhead were collected in Lake Czarne, and one in July 2010 was caught by angling method in Lake Długie in Olsztyn (northeast Poland). The specimens collected in Lake Czarne range from 72 to 154 mm total length (TL) and weighed from 3.9 to 36.8 g. Among measurable characters, the greatest variability was noted in head width, preanal distance, and snout length. The least variability in measurable characters was noted with regard to dorsal fin length, eye diameter, and preventral distance. Because of the adverse impact brown bullhead would have on the indigenous ichthyofauna of the Łyna drainage basin, its spreading occurrence in this region is an ecological and economic threat.
