When studying the ways of structuring social life in general, and the socialization processes of children in particular, ethnicity is often taken for granted as a basic axis along which people distribute themselves into groups. Most anthropological studies describe and classify agents in ethnic terms. This article argues that ethnicity is not always a relevant principle of order in the social fields of children, at least in the ethnographic cases studied. The article provides a critical reflection on the theoretical frameworks and methodologies based on the idea of "ethnic groups" as a starting point for doing anthropological research. (Contains 11 notes.)