Morphological variation among populations of Percina peltata (Shield Darter) from streams in Maryland and Pennsylvania was examined, with particular emphasis on comparing a geographically isolated, lowland population on the eastern shore of the Chesapeake Bay (Choptank River basin) that has been excluded from previous systematic works on other upland populations of this species from west and north of the Chesapeake Bay. Statistically significant morphometric differences were observed between the isolated population and other populations examined, with measurements of eye diameter and the distance between the posterior of the first dorsal fin and the anterior and posterior bases of the anal fin explaining the most variation between populations. No differences in meristics were found. The morphological distinctness and geographic isolation of the Choptank River population of P. peltata, in conjunction with the threatened status of the species in Maryland, suggest special emphasis should be given to conserving this unique component of the biodiversity of the Chesapeake Bay.