Presented are the results from analyzing the dynamics of the ethnic mosaic index of the Russian regions between the population censuses of 1959 and 2010, with a division into two periods: the late Soviet period (until 1989) and the post-Soviet era. In the late Soviet period, the growth of polyethnicity embraced 55 % of the regions of the RSFSR. They were mostly Russian regions that experienced an influx of migrants mainly from Ukraine and Belarus as well as from other Union Republics and a number of national autonomies of Russia. Already then most of the autonomous republics showed a tendency for an increase in the share of the population of titular enthnicity. In the post-Soviet period, in spite of the ongoing decline in the Russian population in Russia, most regions of the country (71 %) experienced a decree of the degree of homogeneity of the ethnic composition of the population. This category included many “Russian” regions where a markedly educed share of the Russian and Belarusian population was never compensated for by the influx of migrants from North Caucasus and the Baltics and the CIS. From the previously extensive territorial massif with increasing polyethnicity in the center of the European part of the country there were only a few regions left in the post-Soviet period, which formed a ring around Moscow. In this situation, Moscow itself began to play the assimilation function with respect to foreign-language-speaking migrants of the Soviet and post-Soviet periods. Most of the national republics showed a steady increase in the share of titular ethnic groups, including because of the indrawal of a given ethnic group to within the boundaries of the autonomy accompanied by an outflow of Russians. Only in a few republics (Karelia, Komi, Udmurtia and Khakassia) and a number of autonomous okrugs did the proportion of the Russian population increase.Keywords: population census, ethnic mosaic index, ethnic composition, titular ethnicity, share of Russians.