Abstract Methodological nationalism is the assumption that nation-states are the relevant units for analyzing social phenomena. Most of the social sciences recognized it as a source of bias, but not the ethics of immigration. Is this field biased by methodological nationalism—and if so, to what extent? This article takes nationalism as an implicit bias and provides a method to assess its depth. The method consists in comparing principles that ethicists commonly discuss when immigration is not at stake with principles advocated in the ethics of immigration. To interpret the results, a distinction between mild and heavy bias is established. When a basic principle in ethics is underdiscussed or absent from the ethics of immigration, the field is ‘mildly biased’. When its negation is commonly advocated, the field is ‘heavily biased’. Here, the method is illustrated with two principles: equal opportunity and reparation. They are common in theories of distributive justice and of corrective justice, respectively. But in the ethics of immigration, scholars often argue for the opposite. Instead of equal opportunity, they implicitly support discrimination based on national origin; instead of sanctions or amnesty for the offenders, scholars plead amnesty for those who they otherwise regard as victims. These preliminary results suggest that the field is heavily biased: methodological nationalism seems to turn ethics into its opposite.