We grew mussels (Mytilus edulis) under two different food regimes and eight population density levels to estimate the joint effects of density and biomass on their growth and survival and to determine the shape of the biomass-density (B-IV) relationship. Mussels were reared for 22 months, between December 1994 and October 1996, in 1-L experimental chambers supplied with natural seston. Growth in shell length, individual wet mass and ash free dry mass (m) decreased with decreasing food availability and increasing population density. Survival was negatively correlated with density but did not differ significantly between food regimes during the first year. Variations in concentration of available food did not alter the effects of crowding on mussels, as judged from the slopes of the body size-density curves. The general patterns exhibited by B-N curves were not consistent with expectations since we found 1) no classical competition-density (C-D) effect as reported in plants at intermediate competition levels, and 2) a slope of -0.648 for m-N curves in both food regimes, which was higher than expected from self-thinning (ST) theory. This value does not support present food-driven ST theory. This study introduces an unusual m-N region which combines properties of both ST and C-D effect.