The census is the most important and unique method of collecting information about the population. The key advantage of the census method, in addition to its inclusiveness, is the one-time collection of various socio-economic and socio-cultural information about the population of the country as a whole and its individual regions. In modern Russia, the census is the only tool for collecting statistical information about the ethnic composition and languages of the country's inhabitants. After several decades of discussions in the expert community about the need to methodologically correct the question of ethnicity in the census questionnaire (providing residents with the opportunity to indicate belonging to several ethnicities and several native languages), statisticians heeded the opinion of ethnologists and, although in a somewhat “compromising” form, added to the census questionnaire information on the possibility to indicate several nationalities. However, the results of sociological studies conducted on the eve and immediately after the census show that, firstly, the biethnors themselves in the overwhelming majority did not know about the possibility of indicating several ethnicities and only a small proportion of them (13%among residents with Russian-Ukrainian ethnicity) intended to list her two ethnicities in the census; secondly, the unprecedentedly low level of direct personal participation of residents in the census (46%) suggests that census information on a significant part of the country's population will be collected from administrative sources that do not contain data on ethnicity. Thus, it is very likely that only about 5-10% of Russian-Ukrainian bi-ethnics will be taken into account in the results of the census. Moreover, a significant number of persons with an “unknown” ethnicity (i.e., enumerated in absentia from administrative sources) in the final census data will create prerequisites for manipulative interpretations of ethnodemographic trends in the country.