An established population of mummichog,Fundulus heteroclitus, in the Susquehanna drainage is reported from Pennsylvania for the first time. Three year classes of this brackish water species were taken on 17 August 1976 from Letort Spring Run, a tributary of the Conodoguinet Creek in Cumberland County. A survey of unpublished records and museum specimens revealed occurrences in Wissahickon Creek, Montgomery County, and Brandywine Creek, Chester County; both Delaware drainage. Presence in these drainages is believed the result of bait bucket introduction.