'Seaside disease' of oysters (Crassostrea virginica)caused by Minchinia costalis (Haplosporida, Sporozoa) produced annual mortalities on the Seaside of the Delmarva Peninsula along the middle Atlantic Coast from Chesapeake Bay to Delaware Bay, USA. The May-June mortalities occurred from 1959 to 1976 without exception; deaths began in late May, peaked in June, and were usually over by July 1. The pathogen developed rapidly from March to May, and sporulation occurred in connective tissues of all organs in May and June. Exposure to a May-June enzootic was required to obtain infections. The pathogen remained subclinical until late winter of the following year. A sympatric pathogen, Minchinia nelsoni , which kills oysters extensively in lower Chesapeake Bay, was present but caused only minor mortalities. Salinities >30 parts per thousand seem to favor M. costalis and inhibit M. nelsoni . Prevalences of both diseases in live oysters or gapers are given for 11 of the 18 years monitored.