Massive mortalities of eastern oysters Crassostrea virginica were observed in 1984 and 1985 along Connecticut's shoreline in Long Island Sound. An archived collection of histological slides was discovered which contained sections of oysters sampled for the late Dr. Sung Feng at the University of Connecticut in response to the oyster mortality event. Oysters from the Hammonasset River, East River, Tom's Creek, and Pawcatuck River were collected for histopathological examination between June of 1986 and December of 1987. A reference sample was collected from Mecox Bay on the Atlantic shore of Long Island, NY. The slide collection consisted of 781 oysters, 59 of which were gapers. Examination of the slides, performed decades later, revealed an epizootic of Haplosporidium nelsoni. The highest prevalence of H. nelsoni occurred in the Hammonasset River in June of 1987 with a 52% prevalence.No H. nelsoni was detected in the reference sample from Mecox Bay, NY. While H. nelsoni plasmodia were detected in 41% of the gapers, 80% of had a terminal infection. Dr. Fengs's archived slide collection demonstrates a wide-spread H. nelsoni epizootic in the mid-1980s in Long Island Sound and establishes the involvement of H. nelsoni in the observed oyster mortalities.