Harbour seals ( Phoca vitulina) in European waters are distributed in clusters. This pattern of distribution is approximately similar from year to year in the Kattegat-Skagerrak area. Haul-out patterns over the season showed distinct differences shallow bottoms and localities surrounded by between localities surrounded by deeper bottoms. A theory, based on certain aspects in the feeding ecology of harbour seals in the area, is presented to explain why seal groups of different sizes are maintained locally. The theory would also explain differences in haul-out patterns over the season at different localities. Alternative possibilities are discussed.