Pylodictis olivaris (flathead catfish) is a large, obligate carnivore native to drainages of the central United States that has been introduced widely beyond its native range. We report on the introduction of flathead catfish into the Delaware and Susquehanna River drainages, which represents the northernmost occurrences among Atlantic drainages. In the Delaware Basin, flathead catfish have been found in the Schuylkill River and the mainstem of the Delaware River. Recent sampling activities in the Schuylkill River suggest successful reproduction and establishment of a viable population there. In the Susquehanna basin, the fish has been found in the lower and central portion of the river mainstem. As in the Delaware Basin, recent sampling indicates that reproduction is occurring in the Susquehanna River. The introduction of this species has the potential to adversely affect ongoing anadromous fish restoration programs and native fish conservation efforts in the Delaware and Susquehanna basins. A cooperative effort will be required to mitigate the effects of this introduction.