Many scholars have asserted that extensive ethnic russification has occurred in Tsarist Russia and the Soviet Union, but little effort has been devoted to making quantitative estimates of the extent of assimilation or of the differences among ethnic groups in tendencies to assimilate. A method of estimating ethnic reidentification in the USSR is described and evaluated. The expected number of survivors to the second census date for each age cohort of a given ethnic group is compared to the reported number of cohort members at the second census date to determine the net number who have changed their ethnic self-identification between censuses. The method establishes relative differences in the propensity to reidentify ethnically among 26 Soviet ethnic groups. Many small ethnic groups in European Russia are found to have very high estimated rates of ethnic russification; non-Russian Slavs—Ukrainians and Belorussians—are found to have fairly low estimated rates of ethnic russification. The implications of the Russian gain in population through ethnic reidentification also are assessed.