In two laboratory experiments decreasing food supply and increasing intra-specific density generally had negative effects on growth and reproduction in the spionid polychaete Polydora ligni. These included longer maturation times at smaller worm size, longer times to deposition of the first brood of eggs and inter-brood period, and decreases in the number of gametogenic segments, egg capsules and fecundity. The number of eggs deposited per capsule was not affected by the treatments. Analysis of field data indicated a significant negative correlation between intra-specific density and fecundity, suggesting that the laboratory results reflect field conditions. Populations of opportunistic infauna, like Polydora ligni. may undergo periods of density-dependent population regulation, and their life histories may be shaped via density-dependent as well as density-independent selective pressures.