<p> <i>Vibrio parahaemolyticus,</i> a human pathogenic bacterium, is a naturally occurring member of the microbiome of the Eastern oyster. As the nature of this symbiosis in unknown, the oyster presents the opportunity to investigate how microbial communities interact with a host as part of the ecology of an emergent pathogen of importance. To define how members of the oyster bacterial microbiome correlate with <i>V. parahaemolyticus,</i> I performed marker-based metagenetic sequencing analyses to identify and quantify the bacterial community in individual oysters after culturally-quantifying <i> V. parahaemolyticus</i> abundance. I concluded that despite shared environmental exposures, individual oysters from the same collection site varied both in microbiome community and <i>V. parahaemolyticus</i> abundance, and there may be an interaction with <i>V. parahaemolyticus</i> and <i> Bacillus</i> species. In addition, to elucidate the ecological origins of pathogenic New England ST36 populations, I performed whole genome sequencing and phylogenetic analyses. I concluded ST36 strains formed distinct subpopulations that correlated both with geographic region and unique phage content that can be used as a biomarker for more refined strain traceback. Furthermore, these subpopulations indicated there may have been multiple invasions of this non-native pathogen into the Atlantic coast.</p>