The theory of demographic transition is a general model of the gradual development of the population of the modern age. The transition process is driven by changes in the mode of production and in social relations caused by industrialization and modernization. The transition model most faithfully describes the demographic changes that occurred in the late 18th and during the 19th century in the highly industrialized, developed countries of the European West. One of the preconditions for the transition process in this area was dynamic urbanization, which was influenced by migration flows mainly on the rural-urban route. These migratory flows were absorbed by the needs of industrial production and accelerated the transition process according to this theoretical model. In the area of European periphery, to which the Croatian historical area also belongs, the process of industrialization and modernization lagged behind in time and intensity. Since the Croatian lands in the second half of the 19th century were part of a complex Habsburg state, the modernization process was determined by the degree of industrialization achieved in the developed industrial parts of the Habsburg state and manifested itself as a "top-down" process. The mismatch between the process of modernization and the process of industrialization in Croatia has resulted in the premature dissolution of the traditional structures of agrarian society beyond the real needs of industrial production. In these circumstances, migratory flows within urbanization could not be absorbed by the real needs of industrialization. In this doctoral thesis, the thesis is that the modernization of migration in the conditions of limited industrialization slowed down the process of demographic transition, ie that it has delayed the beginning of the initial phase of the transition process. The analytical structure of the paper is based, first, on the establishment of two subpopulations of the city of Zagreb, conditionally speaking indigenous and newly settled population, in the second half of the 19th century and, second, on a comparative analysis of the transition of mortality within these populations. The purpose of this analysis is to determine whether mortality of the population of the city of Zagreb can help establish the difference between the structures of domicile and immigrant population and whether, if so, to what extent the mortality of the immigrant population affects the general characteristics of mortality in the city of Zagreb. The dissertation consists of seven chapters arising from the analytical conception. The first three chapters are devoted to the methodological and theoretical impostation, while the remaining three chapters contain quantitative analysis. In the first chapter, Sources and methods, the methodological principles of the analytical procedure are explained. First, the available sources are critically described with an assessment of their applicability in the analytical process. The second part of the chapter describes the statistical methods applied in quantitative analysis and defines the function of these methods in the analytical procedure. In the second chapter, The Theory of Demographic Transition, the historical development of transition theory is critically presented and the historical conditionality of the process of demographic transition is explained in the context of the process of industrialization and modernization. Particular attention is paid to the critique of transition theory to date. The third chapter, the process of modernization and development of Zagreb in the second half of the 19th century, brings the historical context of the research problem. The development of the city of Zagreb is presented through the prism of the modernization process. The modernization process is considered on three levels, first theoretically as a general development model, then at the level of Croatia and Slavonia as a broader social context of which Zagreb is the center and finally as a framework for urbanization of Zagreb in the second half of the 19th century. In the fourth chapter, Survey of Research, a critical review of the current knowledge about the process of demographic transition in Croatia created within the framework of Croatian demohistoriography is given. In three analytical chapters, Analysis of Birth and Nuptiality; Mortality analysis; The transition of mortality by the demostatistic method, the variability in the size of two subpopulations of the city of Zagreb in the second half of the 19th century is precisely determined - residents born in Zagreb and those born outside it. By comparative statistical analysis of annual data on age, sex, residence and religion of deaths in two urban subpopulations, a descriptive analysis of mortality in the city of Zagreb during the transition period is conducted. The social structure of immigrants was analyzed separately, as well as the influence of sociodemographic factors on the mortality of the population of the city of Zagreb. By analysis of the mortality trend of the total population of Zagreb, transition of mortality in the last decade of the 19th century, when mortality rates permanently fell below 30 ‰, is precisely timed. Significant differences in the decline in mortality were found among the analyzed subpopulations. At the annual level, the variations were twice as large in the immigrant subpopulation. Considering that in the inter-census period 1890-1900, which includes the year of mortality transition (1897), the two subpopulations had only 1% of the common variability, we conclude that the mortality trends of these two subpopulations were influenced by socioeconomic factors. The analysis of mortality with respect to the social status of the deceased for each subpopulation separately showed slightly smaller differences. Social differentiation began to differ significantly only from 1897, that is, with the first year of the mortality transition. From this, it is concluded that, in the mortality of Zagreb residents, a structural difference can be observed between residents born in Zagreb and those who immigrated, and that this difference is based on socio-economic factors. With the increasing proportions of the immigrant population towards the end of the 19th century, immigrant mortality began to dominate the characteristics of general mortality. Therefore, it is concluded that migrations to Zagreb prolonged the transition of mortality and delayed the initial phase of demographic transition.