This paper aims at refining the theoretical understanding of the coming-into-being of ethnicities in the light of societal and cultural change, and as a social form of symbolic collectivity. The specific contribution is to the research context of migrants and migrant descendants; wherein conceptual debates on self-perceptions, modes of belonging, group formations and collective subjectivities continue to be at the core of theoretical considerations. In conducting longitudinal fieldwork among Portuguese emigrants in diverse diasporic settings on the one hand, and Portuguese Muslims of Indo-Mozambican origin, on the other, this research inquired into different milieus and generations of people with migration experience in their family histories who share “memories of colonization and migration” (Weber). Over time, generations and migration trajectories, ethnic self-perceptions and membership roles changed among both groupings. The complex settings illustrated here move this preparatory work towards a new analytical concept which I call Ethnoheterogenesis (EHG). Emphasis lies on the genesis and changes of ethnic framing and multiplicity of ethnic memberships.