The census can be understood as a society-wide, systematized and institutionalized interaction, before which actors, in anticipation of the census results, engage in specific activities in the form of debates, talks, and publishing and distributing various informational materials and artefacts. The general purpose of these activities is to persuade census participants to fill in a particular ‘fact’ which is in the interest of the persuading person or institution (the state, minority organizations, and activists). In the context of the Czech Republic, we focus on filling in the questions related to ‘ethnicity’ and ‘mother tongue’ and demonstrate that the census results on ‘ethnicity’ rather than on ‘mother tongue’ influence language policies in the locality. Specifically, we deal with the persuasion campaigns of the German and Polish ethnic minorities.