Freshwater crayfishes are particularly susceptible to non-native species introductions.  Reduction in abundance or outright loss of native crayfish species following the introduction of non-native species has been documented in many areas worldwide.  Species commonly sold as food or bait often become feral and displace native species. Using data on stream crayfishes collected by the Maryland Biological Stream Survey during 1996 and 1997, we examined the decline of the native crayfish, Orconectes limosus following the expansion of a non-native crayfish, Orconectes virilis. This non-native species, first introduced in the Patapsco River Basin near Baltimore, has become widely-distributed in most major river basins in the Piedmont region.  Orconectes limosus has subsequently declined in this region, likely through competitive displacement.  Orconectes virilis has expanded its range in Maryland through a combination of natural dispersal and the inadvertent introduction by local fisherman via bait buckets.  The spread of O. virilis has also been facilitated by stream degradation in Maryland. Orconectes virilis has successfully displaced the native species (O. limosus) in streams having significantly lower forested land cover, lower physical habitat quality, higher nitrate concentrations, and lower biological integrity compared to streams still occupied by the native species.