An epizootic caused by a sporozoan,Minchinia nelsoni, commonly known as “MSX”, began in large plantings of oysters at the mouth of the York River, Virginia, in 1959. In 1960 all public and private beds in lower Chesapeake Bay experienced heavy losses. Commercial plantings in this area ceased in May 1960. Three slightly wet years were followed by three very dry years, which permitted the parasite, MSX, to spread farther up Chesapeake estuaries. After seven years, no important changes in patterns of timing or intensity of activity have been observed in epizootic areas. Distribution of MSX in Virginia for typical years is depicted with an attempt to categorize areas as to amount and frequency of damage to be expected. The history of the epizootic and some effects on the oyster industry are described. Prevalences of MSX and death rates over a seven-year period are compared.