Gammarus daiberi n. sp. andGammarus palustris n. sp. are newly described from estuaries of the Chesapeake and Delaware Bay regions, and further tide-water records ofGammarus tigrinus Sexton,G. fasciatus Say, andG. mucronatus Say are given. A key to American Atlantic species ofGammarus (sens. lat.) is included. In contrast to amphi-AtlanticGammarus faunas of northern glaciated coastlines, theGammarus species of the Middle Atlantic States are North American endemic.