Effects of intraspecific density on growth of the tube-building polychaete Diopatra aciculata (Onuphidae) were examined over a three-month period within a marine worm aquaculture facility. Three polychaete densities (500, 1000 and 2000 worms/m2) were represented within triplicate 0.30 m2 boxes containing late juvenile D. aciculata, sandy sediment and recirculating seawater. Daily food ration per worm was held constant across all density levels. Total length, weight and number of segments were recorded for 20 polychaetes randomly removed from each of nine treatment boxes at weeks 1, 7 and 14. Mean daily growth was higher during weeks 1-7 than during weeks 7-14 for all growth variables at each density level. Polychaetes at the highest density level exhibited lower rates of growth and more broken and/or regenerating posterior segments than those at low density. High D. aciculata density was also associated with reduced dissolved oxygen concentrations and high polychaete mortality (20%). At medium polychaete density (1000/m2), D. aciculata exhibited low levels of apparent stress and high biomass return per unit area, both of which are important considerations in the aquaculture rearing of this species. We suggest that further studies focus on age- and size-related factors contributing to density effects on polychaete growth.