Although natural increase has been recognized as the main driver of postwar urban growth in developing countries, urban transition theory predicts a dominant role for population mobility in the early and late phases of the process. To account for this discrepancy between theory and empirical evidence, I demonstrate the complex role played by internal and international migration in the pattern of urban growth. Using a combination of indirect demographic estimations for postwar Albania, I show that the dominant contribution of natural increase from the 1960s to the 1990s was induced by a limited urban in-migration; this was due to the restrictions on leaving the countryside imposed under communist rule and, thereafter, to the redirection abroad of rural out-migrants. Although young adults in cities also engaged in international movements and significantly reduced their fertility, the indirect effects of rural-to-urban migration attenuated the fall in urban birth rates and postponed demographic aging. In-migrants swelled urban cohorts of reproductive age and delayed the urban fertility transition. Despite a high level of urban natural increase in Albania, I thus conclude that the role of population mobility dominated in the early and most recent phases of urban growth. The results also have implications for our understanding of demographic processes during the second urban transition in developing countries.