We present the first detailed information on breeding marine birds from the southern Lagoon of New Caledonia, southwest Pacific. The area under consideration, 5,000 km², hosts 74 islets varying in size from ca. 0.05 to 30 ha. We found 13 breeding seabird species, of the 23 known to breed in the New Caledonian region. The most significant seabird numbers included large colonies of Black Noddy (Anous minutus, 50,000-100,000 pairs) and especially Wedge-tailed Shearwaters (Puffinus pacificus); the latter species totaling half a million pairs and probably the largest colony in the world. About 5,000 pairs of Roseate Tern (Sterna dougalli) have been found. Several species of conservation concern are also breeding in appreciable numbers, e.g., Fairy Tern (Sterna nereis) and Tahiti Petrel (Pseudobulweria rostrata); both New Caledonian endemic subspecies. However, some species are now very rare (e.g., Red-footed Booby Sula sula), and others have appreciably declined over the last eight years (Fairy Tern, Black-naped Tern Sterna sumatrana and Crested Tern S. bergii). The recent increase in the frequency of visits to islets by people from Nouméa (ca.100,000 inhabitants), resulting from the increase in the number of leisure boats, may be the principal factor responsible for the current trends and threats. Human disturbance seems to be a major threat that may lead to desertion of seabirds on islets, or reduced breeding success.