Marine wood-borers often live in sympatry, sharing deadwood scattered at sea, bothas food and habitat. In this study, carbon and nitrogen isotope compositions were determined totest the hypothesis that the trophic niches of Chelura terebrans, Limnoria quadripunctata, andNototeredo norvagicaobtained from softwood boards maintained in running, unfiltered seawaterare different. Comparison of isotope compositions supports niche partitioning, with N. norvagicaforaging primarily on wood, and crustaceans foraging on decaying wood. Needs and acquisitionroutes for nitrogen determine the trophic behavior of the species. Results presented here are valu-able for assessing the impact of wood-boring species on each other, but also for evaluating theeffect of the separation of carbon and nitrogen sources on the diversity of the interactions betweenco-existing species belonging to the same trophic guild.