An annual phytoplankton cycle in lower Chesapeake Bay, Virginia, is described based on data obtained in 1960 on 24 cruises to five stations. The principal organisms wereSkeletonema costatum, Chaetoceros affinis, C. compressus, Cerataulina bergonii, Chilomonas (?) sp.,Massartia rotundata, Peridinium triquetrum, Gyrodinium aureum (?),Gymnodinium sp.Pyramimonas sp. andCryptomonas sp. Diatoms were dominant in winter while flagellates were more important during warm periods and late autumn. Both groups were of comparable significance, generally, with flagellates being somewhat more important in the York River and diatoms in the bay proper. There were four periods of population maxima and six peaks of species diversity. Diversity appeared to spread upbay from the sea during cold periods and downbay from the rivers during warm seasons. Highest mean population and diversity levels occurred on the western side of the estuary where discharges from several major river systems are received. Despite this indication that environment controls long-term levels of abundance, it could not be demonstrated that nutrients or other specific factors mediated the annual cycle.