Understanding the development and structure of people's concepts of national groups can contribute to an understanding of their behavior in the political arena, including perhaps the recent rise in nationalism and anti-immigrant sentiment. Here, we provide a developmental investigation of concepts of national groups in a sample of 5- to 8-year-old Canadian children (N = 79). Using an extensive battery of measures, we assessed the extent to which children conceive of national groups as socially constructed versus as having deeper, perhaps biological, "essences" that shape their members' physical and psychological makeup. At younger ages, Canadian children tended to essentialize national groups, including in a biological sense. At older ages, the biological conception of national groups subsided, but children continued to view these groups as meaningful and informative. A statistical comparison with 5- to 8-year-old American children's responses to the same measures (N = 70; using data from Hussak & Cimpian, 2019) revealed a great degree of overlap, despite substantial differences between the two countries in how national identity is conceived and described. These findings add an important piece to our understanding of the development of concepts of national groups.