Previous reports of female-female pairings in seabirds concerned surface-nesting species only and the genetic relatedness between the two females has been determined only once. Here, the first observation of a female-female pair (detected by molecular sexing) in a burrow-nesting seabird is documented. In 2003, two female Cory's Shearwaters (Calonectris diomedea) were found incubating the same egg, from which a chick hatched. A genetic (microsatellite) analysis showed that the two females were unrelated. This study provides additional evidence that petrel mating systems may be more flexible than initially thought. Female-female pairing in burrow-nesting seabirds might be promoted by similar factors to surface-nesting seabirds, and might have remained undetected until now due to the secretive habits of burrow-nesting seabirds and the low number of genetic studies conducted.