Haplosporidian parasites have caused significant mortalities in commercially important shellfish species worldwide (Ford and Haskin, 1982;Burreson et al., 2000; Burreson and Reece, 2006) . Haplosporidium nelsoni is the disease agent of the highly pathogenic Multinucleate Sphere X (MSX) disease in the eastern oyster, Crassostrea virginica. The Pacific oyster Crassostrea gigas is thought to be resistant to this disease but reports of H. nelsoni in this species have taken place. The Olympia oyster, Ostrea conchaphilla, is thought to be a possible host species for this pathogen. To date, H. nelsoni has been detected in North America, the Gulf of Mexico ranging from Florida to as far south as Venezuela, in Canada,, Japan, Korea, more recently in China and in Europe it has been reported in C. gigas in France. Haplosporidium amoricanum is another member of the phylum haplosporidia. Unlike H. nelsoni it is not known to have had a detrimental effect on oyster populations and production. H. amoricanum is found in Europe (Netherlands and Spain). This study is the first reporting of H. nelsoni and H. amoricanum in oysters in Ireland.