The global annual usage of plastics has increased dramatically over the last decade. Polystyrene is the fourth most common plastic material produced annually due to its many versatile applications. Consequently, there has been a coinciding increase in polystyrene wastes, much of which makes its way into waterways and oceanic habitats. While plastic debris has been shown to adversely affect many marine species as a result of ingestion and entanglement, little has been studied regarding the effects of small-scale plastic particles (nano and micro) on cellular responses of marine invertebrates. In this study, we investigated the potential for uptake of polystyrene nano and micron sized beads (50nm and 3um) by the Eastern Oyster, Crassostrea virginica, as well as the toxicity of the particles alone and combined with pyrene. Contamination of plastic by PAHs, such as pyrene, which can be toxic to marine life, presents significant issues of concern about impacts on marine species. This research was focused on four key components; 1) the toxicity of nano and micron polystyrene particles, 2) how particle size would affect particle uptake by hepatopancreas (hp) cells in vitro, 3) the difference in uptake of micron and nano particles in vivo between gill and hp cells, and 4) the cellular response of hp tissue after exposure to polystyrene beads alone, as well as polystyrene beads incubated with pyrene in vitro. Generally, this research indicated that oysters accumulate polystyrene beads, with greater intracellular accumulation of nanoparticles in hepatopancreas cells. While polystyrene was generally nontoxic, there was some evidence that nanoparticles could serve as a Trojan horse for pyrene.