Between 1999 and 2003 we collected information on the breeding distribution and abundance of 12 seabird species occurring on nine islands off the coasts of north and central Chile (27°-33° S). The Peruvian booby Sula variegata was the most abundant seabird with a breeding population of ca. 18,000 pairs concentrated in two islands, followed by the Humboldt penguin Spheniscus humboldti with ca. 9,000 pairs, the largest colony being at Chañaral Island with ca. 7,000 pairs. Kelp gulls Larus dominicanus bred at all the surveyed sites in colonies of variable size, ranging from 40 to 2,000 pairs. Peruvian diving-petrels Pelecanoides garnotii and Peruvian pelicans Pelecanus occidentalis bred at restricted sites, but generally in large colonies. Magellanic penguins Spheniscus magellanicus, wedge-rumped storm-petrels Oceanodroma tethys, Neotropical cormorants Hypoleucos brasiliensis, guanay cormorants Leucocarbo bougainvillii, red-legged cormorants Stictocarbo gairmardi, band-tailed gulls Larus belcheri and Inca terns Larosterna inca nested at few sites forming small colonies (from a few to 150 pairs). Two new breeding sites are reported for the Peruvian diving-petrel and nesting of the wedge-rumped storm-petrel is confirmed on the Chilean coast for the first time. Despite protective status, most of the islands showed human disturbance, derived mainly from guano harvesting, egging and tourism. On at least five of these islands we were able to confirm introduced mammals including rats, rabbits and cats. These factors are likely to be detrimental to seabirds and thus demand detailed assessment. Further comprehensive ornithological surveys in other areas are needed so as to improve the scarce knowledge that we currently have on the seabird populations along the Chilean coast.Key words: seabirds, breeding colony, abundance, conservation, Humboldt Current, Chile. RESUMENEntre 1999 y 2003 obtuvimos información sobre la distribución reproductiva y abundancia de 12 especies de aves marinas en nueve islas frente a las costas del norte y centro de Chile (27-33° S). El piquero Sula variegata fue la especie más abundante con una población reproductiva de 18.000 parejas, concentrada principalmente en dos islas. La segunda especie más abundante fue el pingüino de Humboldt Spheniscus humboldti con ca. 9.000 parejas, siendo isla Chañaral la colonia más grande con ca. 7.000 parejas. La gaviota dominicana Larus dominicanus nidificó en todas las islas formando colonias que variaron desde 40 a 2.000 parejas. Yuncos Pelecanoides garnotii y pelícanos Pelecanus occidentalis nidificaron en pocos lugares, pero formando grandes colonias. El pingüino de Magallanes Spheniscus magellanicus, la golondrina de mar peruana Oceanodroma tethys, el yeco Hypoleucos brasiliensis, el guanay Leucocarbo bougainvillii, el lile Stictocarbo gaimardi, la gaviota peruana Larus belcheri y el gaviotín monja Larosterna inca nidificaron en colonias pequeñas (1 a 150 parejas) y en pocos lugares. Se encontraron dos nuevos sitios de nidificación de yunco y se confir...