Scientific research into the occurrence and population density of the Harbour Seal ( Phoca vitulina L.) in the coastal regions of the Netherlands, necessary for any efficacious nature conservancy programme, was started in September 1953 by the author. A reliable calculation of the total number of Harbour Seals in the coastal regions of the Netherlands soon proved to be extremely difficult. Estimations of the number of Harbour Seals occurring in these regions had been made several times before. BROUWER (1927) took the total number of seals in the Netherlands part of the Waddenzee at 1500 and the total number of seals in the estuaries of the provinces of Zuid-Holland and Zeeland at 800. The first number had been arrived at by means of countings in the field, the latter had been computed from the number of dead animals brought in on account of a bounty system existing for more than twenty years. HAVINGA (1931, 1933) also based his most ingenious calculations on the number of animals killed for bounties. He found that, should the total number of seals stay at the same level, the total population should amount to at least 4000 animals, the bounty killings amounting to 1100 animals annually. At the moment HAVINGA published his report, there was no direct evidence of a decrease in the number of seals, but even so HAVINGA obviously felt a slight doubt in this respect. ERNA MOJHR (1952) compared HAVINGA’S calculations with Russian calculations concerning the Harp Seal (Phoca groentandica FABR.). Her conclusions are that only a total number of 8500 animals was sufficient to endure an annual killing of 1100 animals without decreasing. To my opinion ERNA MOHR was mistaken in using these calculations, concerning a species with quite another biology and a shorter span of life than the Harbour Seal. So, taking into account the Harbour Seal only, the various authors came to different estimations in the coastal regions of the Netherlands. Therefore it seemed worthwhile to attempt a more exact census of this species.