My dissertation addressed the issue of metal bioaccumulation in marine deposit-feeding polychaetes that may serve as a conduit for metals between sediments and benthic predators. The ubiquitous polychaete Nereis succinea was used as a model species to study bioaccumulation patterns and processes of arsenic (V), cadmium and chromium (III). Sediments used in my study were collected from three different sites - two in Chesapeake Bay and one in San Francisco Bay. Specific objectives were to understand the relative significance of metal uptake route (water vs. diet) contributing to the total metal body burden accumulated by the worm, and secondly to identify those factors most influencing bioavailability of ingested metal. Hypothesized factors affecting metal bioavailability included the distribution of metal in food, including the geochemical partitioning of metals in various inorganic and organic sedimentary fractions and the metal's subcellular partitioning in algal cells, and the chemical composition of N. succinea's gut fluid composition. Experimentation largely relied on application of radioisotopes (73As, 109Cd, 51Cr) to follow biological processes such as metal uptake by worms from water, assimilation from ingested food and its loss from tissues, and abiotic processes such as binding with various sedimentary fractions. Kinetic parameters such as metal assimilation efficiencies (AE) and loss rate constants (ke) that were generated from aqueous exposure and pulse-chase feeding experiments using live organisms were then used to predict metal accumulation for deposit-feeding polychaetes residing in sediments at two different estuaries - i.e., Chesapeake Bay and San Francisco Bay. Modeling showed that in nearly all cases As, Cd and Cr are accumulated by worms primarily from ingested diet, and secondly that metals loosely associated with sediment particles, represented by the operational fraction carbonex combining exchangeable and carbonate fractions, can be used to predict metal bioaccumulation in N. succinea. Furthermore, multiple regression analysis showed positive relationships between metal AEs and exchangeable and carbonate fractions independently and when combined, whereas other operationally defined fractions, also determined by the sequential extraction procedure (including Fe/Mn oxides, both organic fractions, pyrite and non-extracted residue were found as non-available. Algal As was highly available (AE: 72%), but the bioavailability of As sorbed to goethite was very low (AE: < 3%) suggesting that As bound to labile organic matter is more important than As in abiotic particles as a source for polychaetes. N. succinea showed no difference in the assimilation patterns for Cd associated with algal cells or goethite, but Cr showed the opposite pattern to As, where it was more bioavailable from goethite (34%) than from algae (3%). Bioavailability of sediment-bound As decreased with its exposure time for sediments. Results of experiments investigating the influence of gut fluid on As release from particles showed that while the percentages of particulate As released into gut fluid were higher or similar to AEs, this release was an essential but not sufficient factor for assimilation.