This Data Release makes available measurements of phytoplankton species composition, abundance and cell size made on samples collected in San Francisco Bay (CA) from April 1992 through March 2014. Phytoplankton samples were collected at 31 stations along a 145-km transect where the variability of salinity, temperature, turbidity and nutrient concentrations reflected a broad range of environmental factors that regulate phytoplankton growth and abundance (station map on ScienceBase homepage). Whole water samples were preserved with acid Lugol’s solution, and 2 to 50 ml aliquots were settled in chambers for 6 to 24 hours (Utermöhl 1958). Phytoplankton cells were then counted and identified using a phase-contrast inverted microscope at 125x magnification, all cells greater than 30 μm diameter were enumerated. Cells smaller than 30 μm were counted at 1250x magnification; following the APHA (1989) strip count method, at least 100 cells of the most numerous taxon were counted. Cell volumes were estimated from measured linear dimensions and geometric formulas that varied with cell shapes. Phytoplankton samples representing 923 distinct date, station and depths were included. The 16,442 entries in this dataset include 609 different taxa within 11 functional groups. Diatoms dominated the total biovolume contributed by each functional group (77%) followed by dinoflagellates (14%) and cryptophytes (4.5%). The top 5 species contributing to the summed biovolume in all samples were: Thalassiosira punctigera, Chaetoceros socialis, Ditylum brightwellii, Thalassiosira rotula and Noctiluca scintillans. By frequency of occurrence, the top 5 species were: Teleaulax amphioxeia, Nannochloropsis spp., Plagioselmis prolonga var. nordica, Skeletonema costatum and Thalassiosira eccentrica. APHA. (1989). Standard methods for the examination of water and wastewater, 17th edn. American Public Health Association, Washington, DC Utermöhl H. (1958). Zur Vervollkommnung der quantitativen Phytoplankton-Methodik. Mitt Int Verh Theor Angew Limnol 9:1-38