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A model which predicts changes in heterozygosity as a function of mating structure is applied to population data from Sanday, Orkney Islands. Overall reduction of heterozygosity would be expected to result from mating patterns typical of the population during the periods 1880-1889 and 1950-1959, with the effect substantially more pronounced in the later period. Most of this reduction is attributable to population size. Non-random failure to mate is shown to retard the loss of heterozygosity slightly in the early pattern, and to accelerate it slightly in the late pattern. Avoidance of consanguineous mating has only a modest retarding effect for both patterns. These results are compared to those from studies of two West Indian populations.