This U.S. Geological Survey (USGS) data release provides phytoplankton data collected from Stagecoach Reservoir, Colorado, during the summer months, July through September, of 2017 and 2018. The data were collected by the USGS, in cooperation with the Upper Yampa Water Conservancy District and Colorado Parks and Wildlife. All data are reported as raw calculated values and are not rounded to USGS significant figures. The dataset includes all routine and quality-assurance and quality-control samples collected at the sampling site on Stagecoach Reservoir at Dam (USGS site 401707106495800). Phytoplankton were identified to the lowest possible taxonomic level, and abundance (density) and biovolume are reported.