Prior to 1998, the only published record of a harmful algal bloom (HAB) in South Carolina estuarine or marine coastal waters was a 1988 Gymnodinium breve red tide that originated in the Gulf of Mexico and was transported with the Gulf Stream to continental shelf waters off North Carolina and then southward to South Carolina nearshore waters. In the spring of 1998, a different dinoflagellate (tentatively identified as Peridinium sp.) formed a red tide in Bulls Bay near McClellanville, South Carolina, the first documentation of a red tide localized to South Carolina estuarine waters. In the spring through summer of 1999, the dinoflagellate formed red tides at several sites in South Carolina estuaries (North Inlet, Bulls Bay, Broad Creek/Hilton Head Island), commonly comprising > 95% of the total phytoplankton biomass, and at times reaching > 100,000 cell ml-1. Results from field monitoring of physicochemical and microbial food web properties conducted prior to and during the bloom suggest a potential relationship of bloom formation with elevated dissolved organic phosphate concentrations. These and other aspects of "Peridinium sp.\'s" ecophysiology will be presented, based, in part, on photosynthesis vs. irradiance relationships and 15N uptake kinetics (urea, NH4, NO3) of ambient populations. The recent and recurrent (1998 and 1999) appearance of this widespread (in South Carolina estuaries over 100 miles apart), long-lasting (from spring through early summer), and often intense (turning the water a deep orange color), red tide raises important issues regarding the condition of South Carolina estuaries (e.g. is this phenomenon a signal of changing water quality?). Hypotheses for ecophysiological bases of "Peridinium sp." bloom formation will be presented.