In the current study, the effects of phytoplankton cell size and methylmercury (MeHg) speciation on the bioaccumulation of MeHg by marine phytoplankton were investigated. Volume concentration factors (VCFs) of MeHg were determined in relation to the surface area to volume ratio of the cells for four species of diatom and a cyanobacteria species cultured in unenriched seawater. The VCFs of MeHg, ranging from 7.3 × 104 to 1.6 × 106, increased linearly as the cell surface area-to-volume ratio increased. It suggests that pico- and nano-dominated phytoplankton communities may lead to larger MeHg accumulation than the one dominated by microphytoplankton. MeHg VCFs increased with increasing chloride concentration from 0.47 to 470 mM, indicating that MeHg bioaccumulation is enhanced under conditions that facilitate membrane permeability by the formation of neutral MeHgCl species. Overall results suggest that the size distributions of the planktonic community as well as the seawater chemistry affect MeHg bioaccumulation by marine phytoplankton. © 2012 Wiley Periodicals, Inc. Environ Toxicol 29: 936–941, 2014.