Alnus maritima has a limited and disjunct range consisting of one county in Delaware, three counties in Maryland on the Delmarva Peninsula, and two counties in Oklahoma near the Red River. Distribution maps were compiled from field and herbarium studies. A. maritima and A. serrulata are sympatric on the Delmarva Peninsula where they have an interesting ecological relationship, but in Oklahoma these two species occur in two different parts of the state. The “threatened species” status of Alnus maritima is discussed