"MSX", an unnamed pathogen of oysters, caused an epizootic in Chesapeake Bay which removed from production nearly half of Virginia's private oyster-planting acreage between 1959 and 1961. The organism did not appear in James River seed beds until fall of 1960. A tongue-shaped distribution of MSX was apparently related to influx of salt water along the channel. In 1960-61 and 1961-62, infections of MSX appeared at Wreck Shoal in the middle of the seed area in October, and disappeared the following April coincident with lowest salinities. Infection levels were approximately 30 to 35% each year in populations adjacent to the channel. No appreciable cold-season mortality occurred at Wreck Shoal. MSX was nearly absent from Wreck Shoal oysters during the warm season in summer salinities of about 15 ppt, but at Brown Shoals, with salinities 2 or 3 ppt higher, it persisted through spring freshets and caused summer deaths. From observations for three rather wet years, it is concluded that persistence of MSX infections in the James River seed area depends upon importation of infective material from the saltier waters o Lower James River and Hampton Roads. Also, damage to the seed area will probably be reflected in quality of seed rather than direct mortality. Planting infected seed in high-salinity waters leads to serious losses.