Perkinsus marinus is a major cause of mortality ofCrassostrea virginica along the Gulf of Mexico. The parasite is discontinuously distributed in estuaries and is essentially coincident with its oyster host. Temperature and salinity, but particularly their interaction, are important environmental influences on levels of Perkinsus in oysters. Other environmental and biological factors include, but are not limited to, pollution and other human influences, host nutrition and growth, spawning and reproduction, age and resistance, oyster density and distribution, and disease vectors.