This paper examines the relationship between interethnic marriages and economic assimilation among immigrants in the United States. Two competing hypotheses are evaluated: the productivity hypothesis, according to which immigrants married to native-born spouses assimilate faster than comparable immigrants married to foreign-born spouses because spouses play an integral role in the human capital accumulation of their partners; and the selection hypothesis, according to which the relationship between intermarriages and assimilation is spurious because intermarried immigrants are a selected subsample from the population of all married immigrants. These two hypotheses are analyzed within a model in which earnings of immigrants and their interethnic marital status are jointly determined. The empirical evidence favors the selection hypothesis. Non-intermarried immigrants tend to be negatively selected, and the intermarriage premium obtained by the least squares completely vanishes once we account for the selection.