Using stable isotope analyses, the present study looked at the fractionation of carbon and nitrogen isotopes between bulk sediment organic matter, particulates from the water column, and benthic consumers from the Northwestern Mediterranean continental shelf. Results showed that sedimentary organic matter in the area under study mainly consisted of phytoplankton detritus. In contrast to their available food source, consumers varied widely both in delta C-13 and delta N-15. Meiofauna fed selectively freshly settled organic particles and channeled energy and matter toward the next upper trophic level. Subsurface primary consumers that feed on less readily digestible sediment organic fractions showed enrichments in rare heavier isotopes as high as those of secondary consumers. Depth of feeding seemed to be a determining factor in their isotope fractionation. Within the sediment, infauna were relying on different food items with different isotope compositions, but metabolic pathways probably explained a great part of their C-13 and N-15 enrichment.