This article is devoted to a comparative analysis of ethnic identity in interethnic and mono-ethnic families on the example of St. Petersburg, and is based on data from a sociological survey conducted in 2017. During the study, members of 106 interethnic families were interviewed and 45 mono-ethnic families were analyzed as a control group. The main conclusions of the study include the following. In interethnic families ethnic identity is established, in the case of a mono-ethnic identity it coincides with the identity of mothers, mostly Russians. In general, the influence of mothers on the ethnic self-identification of children is stronger than the impact of fathers. This is due to the dominance of maternal socialization, inherent in the national tradition, and also because most Russian mothers bring up their children in the Russian cultural environment. The attitude towards their ethnic identity in children is more indifferent compared to the sense of “national pride” inherent in most parents. For all categories of the sample, with the exception of fathers from interethnic families, cultural signs of self-identification are more important than others. Fathers of different cultures allot special importance to blood kinship. The main features of ethnic consolidation for all survey participants are cultural traditions and the native language. National identity for all survey participants, except for “non-Russian” fathers, prevails over ethnic self-determination. Ethnic prejudices dominate over religious prejudices. In comparison to Christians, Muslims are more rejecting. The presence of ethnic prejudice is not connected with membership in an interethnic or mono-ethnic family, and is most likely due to the personal attitudes of the respondents.