The aim of this study was to examine social variation (social class, legitimacy) in infant mortality in a core municipality (Viipuri) in Finland from 1819 to 1918. Data were obtained from the Population Change Tables of Finnish, Swedish and German parishes of the town of Viipuri. During the 19th and early 20th centuries the German parish socially had the highest rank and the Finnish one the lowest while the Swedish parish held an in-between position. The study period was divided into the preindustrial period (until 1868) and the period of industrialization (from 1868 onwards). Results of the study indicated a clear association between social class and infant mortality in the preindustrial period; in the period of industrialization the association was also clear but smaller. Legitimacy as a determinant of infant mortality became more marked towards the early 20th century. Industrialization brought with it some factors, such as numerous public health measures, which benefited the legitimate infants and narrowed their social group differences.