Though there is a considerable body of literature on the optimal population problem in a closed economy model there is no such discussion in an open economy setting. A beginning is made in this paper to consider the optimal population and the optimal tariff problems in an open economy model. The optimality rules for the population size and the tariff rate are derived under three alternative formulations of the social utility function. A distinction is made between (a) the small country case and the large country case, (b) the "full optimisation situation" where both the tariff rate and the population size can simultaneously find their respective optimal values and the "restricted optimisation situation" where one of the policy instruments is held fixed at an arbitrary level and (c) the reproduction-based population growth case and the migration-caused population growth case. The sensitivity of the optimality rules to each of these distinctions is explored.