The sedimentary deposits on the Atlantic Coastal Plain in New Jersey, Delaware, Maryland, North Carolina, South Carolina, and Georgia span the Cretaceous/Tertiary boundary. We investigate the ammonites of the Severn Formation on the western and eastern shore of Chesapeake Bay, Maryland, and the Peedee Formation, North Carolina. We describe three ammonite assemblages from the Severn Formation and their associated dinoflagellates, defining three successive ammonite zones in the upper Maastrichtian. The lowest ammonite zone is the Discoscaphites conradi Assemblage Zone. It occurs near the top of the Severn Formation in Prince Georges County, Maryland, just below the Paleocene Brightseat Formation. The ammonite fauna consists of Sphenodiscus pleurisepta (Conrad, 1857), Sphenodiscus lobatus (Tuomey, 1856), Discoscaphites conradi (Morton, 1834), Discoscaphites gulosus (Morton, 1834), Jeletzkytes nebrascensis (Owen, 1852), Glyptoxoceras rugatum (Forbes, 1846), Baculites vertebralis Lamarck, 1801, and Eubaculites latecarinatus (Brunnschweiler, 1966). Dinoflagellates from a sample of matrix include Isabelidinium aff. I. cooksoniae (Alberti, 1959) Lentin & Williams, 1977, which correlates with calcareous nannofossil Zone CC25b, indicating the lower part of the upper Maastrichtian (68.2–67.4 MaBP). The D. conradi Zone is also present in parts of the Corsicana Formation, Texas, the Prairie Bluff Chalk, Alabama and Mississippi, the Peedee Formation, North Carolina, and the Navesink and New Egypt Formations, New Jersey. The next higher zone is the Discoscaphites minardi Assemblage Zone, which occurs in the Severn Formation approximately 6 m below the base of the Hornerstown Formation at Lloyd Creek, Kent County, Maryland. The ammonite assemblage is dominated by Discoscaphites minardi, n.sp., B. vertebralis, and S. pleurisepta, with rare specimens of Sphenodiscus sp., Discoscaphites iris (Conrad, 1858), and E. latecarinatus. A sample of dinoflagellates from the same bed as the ammonites includes Deflandrea galatea (Lejeune-Carpentier, 1942) Lentin & Williams, 1973 and Thalassiphora pelagica (Eisenack, 1954) Eisenack & Gocht, 1960, which correlate with the Neophrolithus freque