The distribution and abundance of Red-legged Cormorants (Phalacrocorax gaimardi) were assessed by visiting 42 localities on the mainland and surveying most of the islands along Peru’s 2,500 km coastline between October 1999 and December 2000. Cormorants were distributed in small discrete groups (Mode = 5 birds, range 1-69) from Isla Foca (5°12’S) to Morro Sama (18°0’S). The southern (56% of the total numbers) and central (34%) coast held a larger proportion of cormorants than did the northern region (10%). Birds were mainly located in unprotected areas, either on islands (6%) or on the mainland (51%). The remainder was found in protected areas, either guano bird islands (27%), guano bird headlands (3%) or within the Paracas Reserve (13%). We counted 658 birds (95% adults, 5% juveniles), but based on bird density, availability of suitable habitats and cliff lengths we predicted a total of 1,803 ± 282 birds in Peru. Red-legged Cormorants built their nests on narrow ledges on vertical rocky walls falling sheer to the sea, located, on average, 17.8 m (range = 3-50, N = 56) above the sea level on open cliffs, high-up in sea cave entrances or on small islets. They have undergone a spectacular decline over the last 30 years. Between 1968 and 1999-2000, the numbers at ten localities in the northern and central coast decreased from 3,229 to 69 birds. It is likely that low numbers recorded in this study reflect the devastating effects of the strong El Niño Southern Oscillation (ENSO) of 1997-98, as numbers prior and after this event at eight southern localities decreased by 73%. Because of the inaccessibility of their nesting and roosting sites and the lack of natural predators, Red-legged Cormorants are apparently not in danger at such sites. However, entanglement in fishing nets, competition for food in inshore waters, pollution, human disturbance and harvesting of kelp banks are potential threats at sea and could affect the population recovery.