This paper draws attention to the influence of season of birth on infant mortality among the Sami and non-Sami population during the nineteenth century. The source material is a set of data files from the Demographic Data Base (DDB) at Umeå University, making it possible to combining age at death (in days), month of death and month of birth during the entire period. For the Sami, the results showed that being born during winter was related to a higher risk of neonatal mortality and being born during summer was related to a higher risk of mortality after 6 months of age. Further, for the Sami, the neonatal mortality showed a U-shaped pattern with a minimum in June-August, whereas the corresponding pattern among the non-Sami was more flat. The findings throw light on vulnerability in two populations sharing the same environment, but diverging by social, economic and cultural factors.