On the eastern seaboard of the USA, populations of the blue crab, Callinectes sapidus, experience recurring epizootics of a dinoflagellate. The parasite, Hematodinium perezi, fulminates in the summer and autumn causing mortalities in high salinity embayments and estuaries. In laboratory studies, we experimentally investigated host mortality due to the disease, assessed differential hematological changes in infected crabs, and examined proliferation of the parasite.  Mature, overwintering, non-ovigerous, female crabs were injected with 103 or 105 cells of H. perezi.   Mortalities began 14 d after infection, with a median time to death of 30.3 + 1.5 d (se).  Subsequent mortality rates were greater than 86% in infected crabs. A relative risk model indicated that infected crabs were 7 to 8 times more likely to die than controls, with decreases in total hemocyte densities covarying significantly with mortality.  Hemocyte densities declined precipitously (mean = 48%) within 3 d of infection and exhibited differential changes in subpopulations of granulocytes and hyalinocytes that lasted throughtout the course of the infection. Crabs that did not present infections after injection (i.e., "immune") did not show hemocytopenia, and exhibited significant long-term (21-27 d) granulocytemia. Detection of the parasite in the hemolymph of infected crabs increased from approximately 30% after 14 d to 60% after 21 d to 100% after 35 d. Plasmodial stages were, however, detectable in histological preparations of the heart within 3 d of infection and increased in number over 5 and 7 d.   Sporulation of the parasite occurred over a short time (at least 4d, after 43 d infection) and did not culminate in the immediate death of the host. The mortality studies indicate that H. perezi represents a significant threat to the blue crab fisheries in high salinity estuaries, may have a greater effect on mature females that move higher salinities to breed.