We describe the current distributions and relative abundances of southeastern Pennsylvania's crayfish; changes in the region's crayfish fauna over the last century; and, where pertinent, the relationship of the current fauna to site-specific characteristics, basin-wide attributes, and exotic crayfish. The crayfish fauna currently inhabiting the region bears little resemblance to the historical assemblage. Whereas historical surveys yielded Orconectes limosus and Cambarus bartonii, both native species, recent collections produced eight species including five exotics. Many areas occupied by exotic Orconectes no longer support O. limosus. Cambarus bartonii was found in a number of invaded systems, but was typically a minor component of the crayfish community and may not be able to persist in those systems indefinitely. The distribution of Cambarus (Puncticambarus) sp., an undescribed member of the Cambarus acuminatus complex, was extremely limited, with populations only found in four streams, all of which are threatened by urbanization and exotic crayfish. Exotic species collections include the first published records for Procambarus clarkii in Pennsylvania and extend the ranges of Orconectes virilis and Orconectes obscurus in the state by > 150 km. These results indicate the need for conservation and management initiatives aimed at preserving the native crayfish that remain in southeastern Pennsylvania.