Two protozoan pathogens, Haplosporidium nelsoni (MSX) and Perkinsus marinus (Dermo) are known to be present in Great Bay oysters. With funds provided by the Piscataqua Region Estuaries Partnership (PREP), the Marine Fisheries Division of New Hampshire Fish and Game Department, (NHF&G) continues to assess the presence and intensity of both disease conditions in oysters from the major beds within the Great Bay estuarine system. Histological examination of Great Bay oysters has also revealed other endoparasites. The American oyster, Crassostrea virginica, may be invaded by a variety of parasites. Two particularly damaging protozoan parasites, Haplosporidium nelsoni (MSX) and Perkinsus marinus (Dermo), have caused widespread high mortalities along the Southern and Middle Atlantic Coast and are now found in New Hampshire waters.