Haplosporidan parasites of oysters have been reported from four continents. Those of the genera Minchinia, Haplosporidium, and Marteilia, which cause serious diseases of oysters, have been intensively studied. Epizootiology of these highly pathogenic species is well known. Life cycles are obscure for all haplosporidans because artificial infections have not been achieved. The high pathogenicity of newly-discovered haplosporidan diseases to native oysters in eastern North America and western Europe may rod:care that these are exohc pathogens paras~t:zmg suscephble oysters not previously exposed to these disease agents. Epizootiology of two haplosporidan pathogens along the middle Atlantic Coast of North America during 25 years of disease activity is discussed. Haplosporidiurn nelsoni sporulates only rarely and its life cycle remains unconfirmed. Resistant oysters were developed in nature and from laboratory breeding. Haplosporidium costale which causes \u27Seaside Disease in high-salinity waters appears to be a more acclimated disease with regular patterns of infection and mortality. Several minor parasites whose life cycles and host species need more study are mentioned
