Pelagibacteraceae, family Enterobacteriaceae, and genus Synechococcus. The microbiomes varied among oysters from the treatment and control groups. The health of C. virginica is impaired by inflowing storm-water runoff from nearby agricultural fields, which carry large amounts of atrazine into estuarine waters. However, the knowledge of oyster-associated bacterial community response to biocidal runoff is scarce. Within the Chesapeake Bay area, marine biologists are interested in uncovering atrazine’s level of toxicity within aquatic systems. We conducted an experiemnt to test if atrazine changes the composition of core microbes found to live in association with C. virginica during non-winter months. As has been reported in other studies (Westlund et. at., 2017; Romero et. al., 2002; A Arfken et. al., 2017; Pierce 2016) our results confirm that oysters maintain similar microbial communities over time. Bacterial species known to be pathogenic to C. virginia and thus related to immunity and defense mechanisms were of particular interest in order to uncover how atrazine is affecting the molecular-bacterial basis of oyster tolerance/resistance to the herbicide’s toxic effects. A novel finding of this study was the observed absence of Vibrio spp. and the subsequent presence of Nocardia spp. in the treatment groups exposed to atrazine. We thus develop a novel hypothesis which states atrazine exposure is driving the evolutionary selection of prokaryotic oyster pathogens in C. virginia populations along the western Atlantic coast of the United States.