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Population structure was investigated for the south-east coast of Tasmania, Australia using the civil registers of births and marriages for the districts of Glamorgan, Spring Bay and Sorell between 1838 and 1950. Because Tasmania was occupied in 1803 by the British to establish a penal colony, the island’s demography has been well documented by a civil bureaucracy and the population structure can be studied for the relatively short but complete period of European settlement. Temporal trends in age and sex structure; census, breeding and effective population sizes; and coefficients of inbreeding and relationship were analyzed. Assortative mate selection for social class and district was also examined in the 20th century marriages. The effects of geographic and demographic factors were consistent with the changes in population structure in the three districts. In Spring Bay in the twentieth century, decreasing effective population size accounted for the increase in the random component of inbreeding and the within-district coefficients of relationship, which occurred despite a general increase in the census size. This anomaly was explained by age-structured migration and demonstrates the importance of calculating the effective population size from the breeding population, rather than as a simple proportion of the census population. In Glamorgan, socio-economic factors underlying class structure were investigated and found to explain the relatively large values for the total and the non-random component of inbreeding. Sorell was the least structured population and the most affected by immigration and marital movement.