Juvenile summer flounder inoculated with Trypanoplasma bullocki via the leech vector or via injection of infected blood and held at ambient temperature become anemic and developed severe ascites. The number of lymphocytes and thrombocytes decreased while granulocytes and progranulocytes increased. During a period of extremely cold temperature (0-1°C) all experimentally infected fish died; no mortality occured in the uninfected control group. Concurrently, dead summer flounder were trawled from the lower York River. These fish were all infected with T. bullocki and all had some degree of ascites. No summer flounder were collected in the lower Chesapeake Bay the remainder of the winter.