The blue petrel (Halobaena caerulea) is a small pelagic seabird breeding on just a few Southern Ocean islands. Although it is known that oceanographic and climatic variability affect mortality rates, individual body condition and reproductive success, very few data are available on the global population status. We assessed the status of blue petrels at Mayes Island, Iles Kerguelen, in 2002 by estimating burrow densities and burrow occupancy using acoustic playbacks and response probability. The mean and highest burrow densities recorded were respectively 0.136 ± 0.026 and 2.3 entrances per m2. Burrow occupancy increased with burrow density but stabilized at c. 0.6 occupied burrows per m2. Taking into account the response probability to playback (0.819) the mean burrow occupancy density was 0.037 ± 0.094 per m2, giving an estimated 142 000 (95% CI around 106 000–179 000) or 148 000 breeding pairs (95% CI around 55 000–241 000) on Mayes Island, respectively with and without taking into account spatial autocorrelation in burrow densities. This suggests that the entire breeding population of blue petrels at Kerguelen is much higher than current estimates, and that Kerguelen and Diego Ramírez archipelagos are key breeding sites for this species.