ABSTRACT: Biotic communities in highly variable, frequently perturbed habitats are usually expected to be structured mainly by abiotic factors. In the highly variable deep-sea hydrothermal vent environment, physical and chemical factors are known to playan important role in limiting the distribution and abundance of species, but the importance of biotic interactions remains largely unresolved. The high density and biomass attained by the vent macrofauna suggest that resource partitioning and competitionmay be significant in these communities. This study of food resource utilisation at northeast Pacific deep-sea hydrothermal vents uses an approach based on stable carbon and nitrogen isotope analyses to characterise trophic interactions between the 3dominant alvinellid polychaetes, Paralvinella palmiformis, P. sulfincola, and P. pandorae. We also examine size structure in sympatric and allopatric populations of P. palmiformis and P. pandorae. Results indicate thatfood resource partitioning occurs both intra- and interspecifically in P. palmiformis and P. sulfincola, and we advocate that this process contributes to explaining their co-existence at very high densities. In contrast, P. pandoraehas a much more restricted trophic niche, overlaps P. palmiformis in diet and is much smaller in size when found in sympatry with P. palmiformis. P. pandorae is the most likely of the 3 species to be affected by intra- and/orinterspecific competition for food, and this may explain the drastic change in the population size structure observed between successive years. Our work indicates that within the limits imposed by environmental conditions, biotic interactions such as foodresource partitioning and competition can be significant factors structuring deep-sea hydrothermal vent communities.