Introduced predators are one of the main threats to island avifaunas. However, the magnitude of their impact in the Mediterranean has not often been studied. This is the case for the introduced alien black rats Rattus rattus, the most destructive predator of seabirds in the Mediterranean. Here, we analyse the impact of black rats on the breeding performance of Cory's shearwater Calonectris diomedea breeding at the Chafarinas Islands, an archipelago with a very high density of rats. An intensive rat control campaign (through anticoagulant poison) was carried out during 1999–2004 at two shearwater sub-colonies with contrasting habitat features (vegetated vs. rocky). Breeding success of Cory's shearwaters increased in proportion to the effort of rat control. Such increase was mainly due to a decline in black rat predation on chicks, while eggs losses remained constant throughout the period. We found a differential effectiveness of rat control in each sub-colony. The increase in breeding success after rat control was higher in the sub-colony with lower breeding success (the vegetated habitat), but this parameter never reached the values shown by the other sub-colony (the rocky habitat). Our results suggest that habitat characteristics of each case of study must be taken into account when designing and evaluating specific rat control programs.