MSX, a pathogen of oysters (Crassostrea virginica), produced a drastic epizootic in high—salinity areas of Chesapeake Bay from 1959 to 1963. The patterns of infection and mortality were determined by imports from disease—free seed—oyster areas. Winter and spring imports became infected in early summer and began dying in late summer. Late—summer imports apparently became infected promptly but infections remained subclinical until the following May. Death rates were highest during warm months but losses occurred throughout the year. MSX was assigned as the cause of a high percentage of deaths by sampling live and dead oysters. Mortality for the first and second years after import was usually 50 to 60% annually. A fungus disease caused by Dermocystidium marinum was also prevalent in some areas. Prevalence of MSX did not decline as oyster populations were decimated; hence nearly half of Virginia's private beds have been forced out of production.