Hematodinium perezi is a lethal parasitic dinoflagellate that lives in the hemolymph of brachyuran crabs. The parasite is found along the eastern seaboard of the USA where it occurs in epizootics in the commercially important blue crab, Callinectes sapidus.  Crab mortalities associated with the disease occur in high Salinity waters, typically in poorly draining estuaries. The parasite is prevalent in the seaside bays of the Delmarva Peninsula in the spring and fall, and spreads to the lower reaches of Chesapeake Bay in the fall.  In October 1996, the prevalence of the disease along the Virginia portion of the Delmarva Peninsula varied from 20 - 50% in legal crabs. Lower prevalences (1-10%) were noted for crabs caught between Cape Henry and Cape Charles, i.e. the mouth of the bay. In November, the prevalence is generally low during the prebreeding and ovigerous seasons. In Spring and Fall, 1997, the disease had a higher prevalence in the coastal bays and creeks. Infected crabs frequently show signs of weakness and lethargy, and often die due to stress-related handling from fishing. The parasite proliferates to extremely high densities in the host (up to 100 million parasites/ml of hemolymph) over 3 to 6 weeks. The hemolymph undergoes radical changes as evinced by its lack of clotting ability, and marked discoloration. Hemolymph levels of total proteins and acid phosphatase activity change with infection, and indicate a gradual decline in the hosts metabolic resources. The effects of other species of Hematodinium on several crab and lobster fisheries, and data from the present study indicate that H. perezi may have a significant impact on the coastal blue crab fisheries along the Atlantic seaboard of the USA.