Parasitic oyster diseases can be fatal under temperature and salinity conditions favorable to the parasites. Haplosporidium nelsoni (the cause of MSX disease) and Perkinsus marinus (which causes Dermo) have been serious pathogens of native Chesapeake Bay oysters since the late 1950s. Samples of oysters (including planted individuals) from each restoration site are collected annually in June, August, and November, and each individual is examined for the presence of H. nelsoni and P. marinus. Prevalence (percent of the population infected) of H. nelsoni and infection intensity is determined by histopathological analysis of oyster tissue samples. Prevalence of P. marinus within a population is determined by culturing tissue samples from each animal and allowing the parasite to form hypnospores, large cells with very thick walls, that can be stained and easily visualized microscopically. Weighted incidence (Mackin 1962), a measure of disease activity that incorporates both prevalence and intensity data, is determined for each disease at each restoration site. Results will provide one measure of the likely survival of restoration site reef oysters in the months following each survey.