To clarify whether microplastics contribute to elevated bioaccumulation of methylmercury (MeHg) in aquatic organisms, we studied the sorption pattern of MeHg on polystyrene beads (PBs) and evaluated MeHg accumulation, via uptake of MeHg-adsorbed PB, in the oyster Crassostrea gigas. MeHg-cysteine conjugates were added to seawater at 10, 100, and 1000µg/L as Hg. Polystyrene beads (φ = 0.02, 0.2, and 2µm) were immersed in the seawater for 24h. The concentrations of total mercury (T-Hg) adsorbed onto the PBs were then measured using the reduction vaporization method. T-Hg concentrations for the PBs with diameters of 0.02, 0.2, and 2µm were 10.6 ± 0.4, 1.8 ± 0.1, and 1.3 ± 0.1ng/mg-PBs, respectively, when immersed in 2mL of MeHg-added seawater (100µg/L as Hg). Thus, the adsorption efficiency of MeHg onto PBs was higher in the presence of smaller diameter PBs. Next, 1mg of PBs immersed in 2mL of seawater containing 100µg/L of MeHg for 24h was added to an oyster tank containing 1L of seawater. The T-Hg concentration of the oysters was measured after 6h of exposure. No significant difference was found in the T-Hg concentration of oysters in the presence of PBs (0.30 ± 0.01 to 0.37 ± 0.05ng/mg as dry weight) with MeHg and in the absence of PBs (0.36 ± 0.03ng/mg as dry weight). Our results suggest that the presence of PBs in seawater has little effect on MeHg uptake by oysters.