A protozoan parasite, Perkinsus marinus, has been responsible for infection and mortality of eastern oysters, Crassostrea virginica, since before 1950. Studies on the course of infection intensity (incidence) in individual animals have been restricted by the need to sacrifice animals for diagnosis, so quantitative association of disease intensity with environmental conditions and individual survival has not been accomplished. A recently developed hemolymph assay provided the means to quantitate infection intensity from live oysters. Application of this technique demonstrated progression of P. marinus intensity in Gulf of Mexico oysters maintained in laboratory aquaria in fed and unfed conditions at different test temperatures (18 - 27 C) and salinities (6 - 36 ppt). In one experiment, incidences over eight weekly samplings were 10 (sup 0.09) hypnospores/ml hemolymph/week for low temperature/low salinity conditions and 10 (sup 0.36) hypnospores/ml hemolymph/week for high temperature/high salinity conditions. Temperature was more influential than salinity in P. marinus incidence and oyster mortalities.