Incidental catch or bycatch represents a significant threat for the conservation of seabird populations. The western Mediterranean is an important fishing area where the Spanish pelagic and semi-pelagic longline fleet targeting swordfish (Xiphias gladius), bluefin tuna (Thunnus thynnus) and albacore (Thunnus alalunga) operates. Bycatch of these fisheries includes several seabird species. Given the importance of conservation of the bycatch species (marine mammals, turtles, sharks and seabirds), an on-board observer program was implemented by the Spanish Oceanographic Institute (IEO); this included collecting data on effort and catch, as well as weight and number of individuals of the main bycatch species. The aim of the present study is to report data on seabird bycatch collected by the on-board observer program of the IEO in the Western Mediterranean. Data on seabird bycatch were collected for the period 2000-2008, throughout the year. Six longline gears targeting large pelagic fish were identified operating in the area of study, but only three had an effect on seabird species. Differences in catch per unit effort (CPUE, birds per 1000 hooks) for each gear, as well as their effect on particular seabird species, are reported in this study. A total of 4 786 466 hooks were monitored, which yielded 182 seabirds belonging to 7 different species. The average CPUE for the studied period was 0.038 birds per 1000 hooks. Cory’s shearwater (Calonectris diomedea) and yellow-legged gull (Larus michahellis) were the species the most highly represented in the catch. In contrast, Balearic shearwater (Puffinus mauretanicus) was only present in the longline fishery targeting albacore (CPUE = 0.005 birds per 1000 hooks). Our results suggest that Cory’s shearwater is the species the most affected by the longline fishery in the Western Mediterranean, probably due to its biological characteristics, and corroborate the well-established downward trend in its population.