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Abundance data from a marine sand flat in the Pacific Northwest reveal a strong negative correlation between abundances of the spionids Pygospio elegans and Pseudopolydora kernpi, and hence suggest that competition is occurring between these two deposit-feeding polychaetes.The data also suggest that the specific winner of a competitive encounter is determined by the initial densities of the two species. A density-dependence experiment was performed in the field to test this hypothesis, in which the density of each species was independently varied over four densities in experimental containers. Immigration, emigration, and mortality combined to cause Pygospio densities in all experimental treatments to converge to ambient density after 8 wk. After 8 wk, Pseudopolydora densities were converging on the ambient density; the densities of some experimental treatments were significantly different from each other, but in no case did an experimental density differ significantly from ambient. The rate of convergence was strongly density dependent in both species. In four different experiments, conducted for I, 2, 4, and 8 wk, Pygospio survivorship was strongly affected by its own density but was weakly affected, if at all, by Pseudopolydora density. Pseudopolydora survivorship was affected only by its own density except for the 1-wk experiment, in which Pygospio density also had a significant effect. The field results conflict with the results from a laboratory experiment where Pygospio rapidly migrated away from Pseudopolydora but not from conspecifics. I propose a spacing hypothesis which predicts that Pygospio relocates as often as necessary to minimize contact with Pseudopolydora. Static spatial data from the field provide some support for the hypothesis.In the laboratory, Pygospio underwent asexual reproduction only at low densities and in the absence of Pseudopolydora. This response is interpreted as an adaptation for the rapid utilization of resources.