Cats (Felis catus) were probably introduced to Reunion Island (Western Indian Ocean) in the seventeenth century and feral populations are now spread over all anthropogenic and native habitats. The diet of feral cats living in the breeding habitat of Barau's Petrel (Pterodroma baraui), an endemic and threatened seabird of Reunion Island, were studied. Results from the analysis of 217 scat (333 prey items) showed that Barau's Petrel were the most common prey of feral cats, followed by introduced rodents. Numerous dead birds at breeding colonies that had been killed by cats were found, 58% of the birds were adults. Given the high sensitivity of the population growth rate of a long-lived seabird to any additive mortality of adults, these results are particularly worrying. As this species is also threatened by massive light-induced mortality of fledglings, it is highly likely that this population is declining. A control of cats at breeding colonies is urgently needed to save this species from extinction.