Changes in the diet of the Peruvian Diving Petrel Pelecanoides garnotii in La Vieja Island were determined from 118 stomach contents collected by the water-offloading technique between 1997 and 2000. Fish were present in 67.8% of samples, crustaceans in 65.3% and squid in 7.6%. By mass, Peruvian Anchovy Engraulis ringens (33.9%), the small krill Euphausia mucronata (26.8%) and Squat Lobster Pleuroncodes monodon (24.3%) were the most important prey species. Other important prey were Mote Sculpin Normanichthys crockeri and fish larvae. High monthly variation in the main prey species suggests an opportunistic feeding behaviour associated with prey availability. Sea-surface temperature anomalies were negatively correlated with diet diversity, suggesting that warm conditions decrease prey options. An increased consumption of anchovy occurred between 1997 and 2000, smaller in size than exploited by the legal fishery. There was no overlap in anchovy size between those caught by the fishery and those consumed by diving petrels, discouraging the hypothesis of direct competition. Nevertheless, other factors related to fishery and El Niño events should be evaluated to assure the conservation of this globally Endangered seabird.