Vibrio vulnificus is an important human pathogen, which occurs naturally in estuarine waters and in large numbers in oysters. The levels of V. vulnificus in shellfish vary greatly, and the causes of this variation are not well understood. A protozoan parasite, Perkinsus marinus, is a pathogen of oysters, which was earlier suggested to increase susceptibility of oysters to bacterial infections. A goal of our study is to determine whether the presence of P. marinus plays a role in determining the incidence or magnitude of V. vulnificus infection in oysters. We also used fluctuating asymmetry (FA) of shell weight as an indicator of developmental instability and thus of microbially-induced physiological stress in oysters. We harvested 155 adult oysters (Crassostrea virginica) from a small environmentally homogeneous site in North Carolina, and examined them for V. vulnificus and P. marinus infections. V. vulnificus and P. marinus were detected in 88% and 73% of the oysters, respectively. The incidence of the two pathogens in these oysters was independent, with no greater tendency to infection by V. vulnificus in oysters infected by P. marinus and vice versa. V. vulnificus loads per g tissue weight varied by over four orders of magnitude in oysters of the same size, weight and age, which were harvested from this single habitat. V. vulnificus loads were found to be unaffected by age of oysters (and thus by the length of exposure to this pathogen). On the contrary, there was a significant decrease in both incidence of V. vulnificus infection and tissue loads of this bacterium with the host weight, possibly indicating growth dilution. Infection with V. vulnificus or P. marinus did not affect FA of shell weight in oysters. Overall, our findings suggest that infection by P. marinus does not predict V. vulnificus loads in C. virginica, nor did oysters infected with either of these pathogens exhibit physiological stress that results in developmental instability.