Within the framework of a larger project on the ecology of Posidonia oceanica in a bay off N.W. Sardinia (Porto Conte Bay), benthic samples were collected in a continuous Posidonia bed along a transect at 1 m, 5 m, 15 m and 25 m depths, by means of two different methods used for sampling the leaf stratum (hand-towed net) and the rhizome stratum (air-lift sampler). Samples were taken first in February 1984 and repeated in September of the same year. Here reported is the distribution and structure of polychaete populations collected in the rhizomes and a comparison with those collected from the leaf stratum. A total of 1,498 worms, belonging to 132 species, were collected (79 species on the leaf-stratum, 101 on the rhizomes, 41 in common). Species richness and abundance were higher in the deeper samples, higher in February than in September and in the rhizome layer than in the leaf layer. A structural analysis (Factorial Analysis of Correspondence) showed a separation of the leaf samples from the rhizome ones, although polychaetes of the leaves do not represent a separate community but rather an impoverished assemblage of the rhizome population, selected against the more stressing environmental conditions occurring on Posidonia leaves. In both layers and in both seasons a zonation along the depth gradient occurred with a discontinuity, particularly evident in summer, between the shallow and the deep samples that represent two different polychaete communities. Spatial and seasonal differences were more pronounced in the shallower stands of the bed than in the deeper ones where a more stable environment accounted for higher species richness and minor fluctuations of species abundance. A total of seven main feeding categories were identified. The feeding guild analysis revealed the dominance of microherbivores mainly at the deeper station, while macroherbivores were more abundat at shallower depth, especially in September. However, also microcarnivores were relevant and these data suggested that polychaetes may play an important role within the Posidonia food web, both as plant and animal consumers as well as favourite prey of higher predators.