Abundant Rangia clathrodonta and Corbicula densata, bivalves indicative of brackish- to freshwater environments, occur in the Upper Yorktown Formation (Pliocene) at three localities in Williamsburg, Virginia. Published models of Yorktown deposition do not fully explain the presence of these brackish taxa in an otherwise marine setting. This study documents these brackish taxa and quantifies the paleosalinity of the environment. The lithology, mineralogy, and fossil content of stratigraphic sections at the three localities were described, and bed thicknesses were measured. 18 five kilogram bulk samples were collected from the beds containing the brackish fauna and from adjacent beds at each site. Half of each sample was washed and sieved through 2.80 mm, 1.00 mm, and 63 micron sieves. The fossils were sorted and identified. The brackish species are found in association with marine taxa, including Plicatula, Glycymeris, and Turritella. Bioturbation in the form of macrofaunal burrows further complicates the stratigraphy. Shells of both brackish and marine molluscs were selected for Sr, C, and O stable isotope analyses, provided by Dr. Eduard Reinhardt at McMaster University, and the results used to quantify paleosalinity. Ostracodes were separated from the fine sediment. Species? salinity affinities were used to constrain paleosalinity limits, wish assistance from Dr. Thomas Cronin at the United States Geological Survey in Reston, Virginia. Brackish and marine bivalves show similar degrees of abrasion and dissolution. Preliminary isotopic analyses suggest that brackish conditions existed for a time, but that the fauna was subsequently mixed with a more marine fauna. Further research and analyses are required to accurately determine the paleoenvironmental conditions.