The application of racial categories to the Jews by Zionist physicians and anthropologists in the first half of the twentieth century has been the focus of several recent studies. In these studies 'nationalism' serves as the primary explanatory framework for Zionists' embrace of racial concepts. This article focuses on the discourse of Zionist men of science, both in Europe and in Palestine, concerning the repercussions of mixed marriages on Jewish racial qualities. It argues that Zionist racial discourse, and eugenic discourse in particular, cannot be interpreted in terms of 'nationalism' alone. In some contexts 'race' was used to establish Jewish unity, whereas in others it was used to establish diversity and hierarchy among Jewish groups. This contradictory use of 'race' is explained as stemming from the ambiguity of racial categories and from the tension between a nation-building project and a cultural project of Westernization which took place in the context of Zionist colonization in the Orient.