An account is given of the mortality of Ostrea edulis at the Tal-y-foel oysterage from 14 days to 11½ years after metamorphosis. In the first year after settlement most of the deaths occur in the late summer, autumn and spring, and few during November-February. A survival curve has been drawn up and shown to correspond with Pearl's ‘positive skew rectangular type’. On the average, from 1000 14-day-old spat, 78 survive for 6 months, 18 for 18 months and 7 for 11 ½ years. At this oysterage the major predators are rare or absent, and the cause of most of the mortality is unknown.