Coastal marine sediments are important sites of methylmercury (MMHg) production, and dissolved efflux provides an important source of MMHg to near-shore, and possibly off-shore, water columns and food webs. We measured the flux of MMHg across the sediment−water interface at four stations in Boston Harbor that span a range of infaunal population densities and bioirrigation intensities. At each station we carried out total MMHg flux measurements using core incubations and collected near-surface pore waters to establish MMHg gradients for diffusive flux calculations. The flux cores were also imaged by CT scanning to determine the distribution of infaunal burrows, and pore−water sulfide and 222Rn profiles were measured. Total MMHg fluxes, measured using core incubations, ranged from −4 to 191 pmol m−2 d−1, and total MMHg fluxes were strongly correlated with burrow densities at the stations. Estimated diffusive fluxes, calculated based on MMHg concentration gradients below the sediment−water interface, were much lower than total fluxes at three of the stations, ranging from 2−19 pmol m−2 d−1. These results indicate that MMHg exchange may be significantly enhanced over molecular diffusion in bioturbated sediments. Furthermore, burrow density provides a strong predictor of total MMHg flux. Pore−water exchange of both dissolved MMHg and 222Rn, a naturally occurring pore-water tracer, increased across the range of observed burrow densities, suggesting that the presence of burrows enhances both MMHg production and flux.