This U.S. Geological Survey (USGS) Data Release provides the phytoplankton tally sheet, including photomicrographs, for samples collected from eleven large river sites throughout the United States, from June through September 2017. All data are raw tallies, not calculated abundances or concentrations. The dataset includes all routine and quality assurance/quality control samples collected as part of a National Water Quality Assessment Project pilot study to describe the potential for cyanobacteria and cyanotoxin ocurrence in the Nation's large rivers. Phytoplankton were identified to the lowest possible taxonomic level and abundances (density reported as natural units) are reported.