An intensive investigation was made of the seston population of the upper Chesapeake Bay from 1 November 1965 through 30 March 1967. During the period of high spring runoff the seston population of the upper Bay was closely linked to the major source of new sediment, the Susquehanna River. Throughout the remainder of the year however, the concentrations of seston were greater within the Bay north of 39° 12′ N (Tolchester) than in the mouth of the Susquehanna River or farther seaward in the estuary, and the concentrations of seston were determined primarily by resuspension and by the net non-tidal estuarine circulation which traps much of the seston within this segment of the Bay. Seaward of 39° 12′ N the concentrations of seston decreased quite sharply throughout the year.