The following is an account of plankton measurements obtained during a cruise2 of the research vessel "Atlantis" to the Mid-Atlantic Ridge between July and September 1947. It is primarily a study of plankton in the great central Atlantic eddy known as the Sargasso Sea. For comparative purposes, a small amount of material was obtained from New England coastal waters, the so-called slope water area lying off the edge of the Continental Shelf, and the Gulf Stream. An August cruise by the "Balanus" assisted in gathering this subsidiary data. The material to be described consists of phytoplankton assays by the plant pigment method of Harvey (1934), measurements of zooplankton collected by means of Clarke-Bumpus (1940) samplers, and experimental estimates of phytoplankton photosynthesis and zooplankton respiration. The plankton work was accompanied by physical and chemical measurements that will be mentioned briefly here and will be described more fully in another publication.