Advocates of no-take marine reserves emphasize their conservation benefits. Critics counter that reserves would decrease fisheries yield. Analysis of a spatially explicit harvesting model, however, shows that no-take marine reserves are always part of an optimal harvest designed to maximize yield. The optimal harvest generates a spatial source–sink structure with source populations placed in reserves. The sizes and locations of the optimal reserves depend on a dimensionless length parameter. For small values of this parameter, the maximum yield is obtained by placing a large reserve in the centre of the habitat. For large values of this parameter, the optimal harvesting strategy is a spatial ‘chattering control’ with infinite sequences of reserves alternating with areas of intense fishing. Such a chattering strategy would be impossible to actually implement, but in these cases an approximate yet practicable policy, utilizing a small number of reserves, can be constructed.