Fiddler crabs are one of the principal agents of bioturbation in intertidal salt marshes. The physical, chemical, and biological properties of sediments can be modified by fiddler crab burrowing activity. This study examined the effect of fiddler crab burrowing on sediment reworking and the distributions of 210Pb and 137Cs in salt marsh sediments at North Inlet Estuary, South Carolina. Fiddler crab burrow density, turnover, and volume were measured along a transect from the forest to the creek bank. Burrow density ranged between 40 and 300 burrows m-2 with highest densities at the creek bank. Sediment reworking is related to burrow turnover, density and size. Sediment reworking rates ranged between 4.4 × 103 and 5.7 × 104 cm3 m-2 y-1. Excess 210Pb and 137Cs profiles indicated that fiddler crab burrowing mixed the top 8 to 15 cm of sediment. Direct field measurements of burrow density, turnover, and size were used as input to a modified version of the regeneration model of Gardner et al. (1987) to assess the effect of fiddler crab bioturbation on 210Pb profiles. The modification takes into account the filling of abandoned fiddler crab burrows from both the infilling of surface sediment and the collapse of burrow walls. Model results were in good agreement with the observed 210Pb distributions in the sediments. Overall the results of this study suggest that fiddler crabs directly influence sediment composition and biogeochemical cycles in salt marsh systems.