We investigated the occurrence of harbour porpoises in the North Sea off Schleswig-Holstein by conducting aerial surveys with emphasis on calves. Fourteen per cent of the sighted porpoises off the islands of Sylt, Amrum and southern Rømø were calves, whereas the average calf proportion for the whole North Sea as established during the 1994 SCANS survey was only 5.4%. The significantly higher proportion of calves off Sylt and Amrum indicates that these coastal waters are used as a preferred calving ground for harbour porpoises. Analysis of stranding data showed that the identified calving ground does not extend south of Amrum and that harbour porpoise calves are present off Sylt and Amrum all year round. The combination of sighting and stranding data demonstrates the special importance of the investigated area for harbour porpoises as a calving and nursing ground. The area should therefore be protected.