We have examined the relationships between Cd ion activity [Cd2+], in seawater, Cd accumulation and subcellular distribution and growth in the polychaete Neanthes arenaceodentata. Organisms were exposed for 3 weeks to a range of [Cd2+] in a Cd-chelate buffer system. Cadmium accumulation and growth were monitored weekly for each exposure group and subcellular Cd distributions were determined at the end of the 3-week period. We found Cd associated with all of the subcellular fractions except the very low molecular weight ligands. Total Cd accumulation was greatest at day 7 and decreased over time in all but the highest [Cd2+] where it remained constant. For each point in time, however, there was a linear relationship between total Cd and [Cd2+] in seawater. Linear relationships were also observed between [Cd2+] and Cd loading in each subcellular ligand pool. Specific growth rates varied with both [Cd2+] and time in a nonlinear manner.