This manual contains a key to 15 families of freshwater and marine amoebae, of which one the Echinamoebidae, does not contain a known marine species. Diagnostic features for 49 genera, of which 34 include marine species, also are given. Descriptions and illustrations for 76 species of marine amoebae and an annotated systematic list are provided. The basic key is designed to assist the user in the identification of recognized species of marine amoebae that have been described from waters of the northeastern United States. However, certain well-known families and genera of freshwater forms are included to assist in their identification should they be discovered in seawater in future investigations. Information also is provided which includes comments on the general biology of the Amoebida, and techniques for microscopic observations and laboratory cultivation of many species. Most of the amoebae described in the key are free living, but a few are parasitic and known to be of considerable economic importance. One new free-living species, Vexillifera minutissima, was discovered in Chincoteague Bay, Va., and is described herein for the first time.