Oysters from the major producing area of Louisiana were found to be 100% infected with the gregarine parasite Nematopsis ostrearum; those from a check station in Pencacola Bay, Florida, were found to be 95.1 % infected. In Louisiana., the population of N. ostrearum was found to be higher in the area east of the Mississippi River. N. ostrearum infections in oysters increased significantly during summer months in areas of relatively low oyster mortality. It was concluded on indirect and negative evidence that N. ostrearum was not a controlling factor responsible for the condition of oysters or for their extensive mortalities in Louisiana waters.