Purpose and scope: The purpose of this study was to determine primary productivity and respiration of the fresh, tidal Potomac River. The data will be used by the authors and other researchers involved in modelling chemical and biological properties to calculate phytoplankton growth rates. Methods are documented in detail so that the report could serve as a reference. The report covers the period from May 1980 through August 1981. Five stations were selected for study that are representative of biologically important reaches of the fresh, tidal Potomac River.