The European Storm-petrel Hydrobates pelagicus is a monogamous and long-lived species with a low reproductive rate. Its nest sites are located in natural cavities under rocks or in burrows excavated by other species. Data obtained in the Molène archipelago, western France, have been analysed to study nest site fidelity and mate fidelity, and to study their relationship with breeding success and sex. Breeders were ringed and breeding success assessed from 2001–18. The influence of age and the weight of breeders on reproductive success has also been studied. Individuals were very faithful to their nest sites (94%) and to their mates (95%) from one year to the next. Breeding failure in the year t was negatively correlated with nest site fidelity and with mate retention in year t+1. Nest site fidelity was not influenced by the sex of breeding birds. Change of nest site or mate had no significant impact on the breeding success in the same year. There was a positive effect of the weight of incubating adults on reproductive success. High rates of nest site fidelity and mate fidelity may indicate good population status, with breeding habitat and mates of good quality.