Pseudonereis gallapagensis and Halosydna johnsoni coexist in the mussel bed habitat of a rocky shore in Peru. There are 2 mussel beds: the Semimytilus algosus bed which extends widely between the low and mid intertidal, and the Perumytilus purpuratus bed which occurs as a narrow band in the mid-high intertidal. Despite seasonal fluctuations in total population density, the density of large polychaete individuals was stable and the total population biomass was of similar magnitude in the 2 species. Size-frequency distributions followed a similar pattern through the year, with recruitment of young seen in the austral spring. Different size groups of Halosydna had similar levels of dispersion throughout the year, but Pseudonereis showed a wide variation in dispersion among size groups, with large sizes demonstrating more strongly aggregated patterns. Overlap in space between the species was smaller than that within either of the species. Interspecifically, the pair involving larger size groups tended to have lower values of spatial overlap. The Semimytilus bed supported more population biomass of both polychaete species than the Perumytilus bed, though the density of Pseudonereis was not significantly different between the 2 habitats. Large individuals (of Pseudonereis, in particular) were scarce in the Perumytilus bed. The mussel bed is a complex habitat with varying sizes of interstitial space, which is considered to buffer competitive interactions between individuals of different mobility and body sizes and to enhance coexistence of the free-ranging polychaetes. from Authors