A major source of social inequalities. Its fluid and multidimensional nature is extensively discussed in theoretical and qualitative work but is less often studied quantitatively, partly due to the lack of appropriate data. In this article, we conduct a quantitative analysis on ethnic fluidity and multidimensionality: We empirically demonstrate that one’s ethnic self-identification may change if not aligned with relevant others’ perceptions. Using longitudinal models on data from 27 freshly formed Hungarian secondary school classes of Roma and non-Roma adolescents (N=784) and a dyadic measure for ethnic perceptions, we disentangle the two-way relationship between ethnic self-identification and ethnic perception of classmates. We find that students perceived as Roma by more classmates are more likely to develop and maintain a Roma self-identification and, independently, less likely to develop and maintain a Hungarian one. Furthermore, a Roma self-identification increases one’s likelihood to be perceived as Roma by others, and independently, a Hungarian self-identification decreases it.