ENIn 1898, a questionnaire was sent the parish clergy on behalf of Antanas Baranauskas, Bishop of Sejny, in which, among other things, they were asked about the local languages used in church practice. An analysis of the clergy’s answers shows that at the end of the nineteenth century, there were not many potential parishes in the diocese of Sejny or Augustów where conflicts over the use of languages in the additional services could arise. The Lithuanian language was well established in most Lithuanian parishes or in those with few Polish believers. The most important source for studying the causes and development of linguistic conflicts in parishes is the parishioners’ petitions written to their spiritual superiors and to the civil authorities. These petitions include demands of each side, explain the situation, and ask for a change in the situation with regard to one language or the other. In the appeals written by the Lithuanian believers, the justification of the introduction of the Lithuanian language in additional services was that the majority of the believers in the parish were of Lithuanian nationality and that most of the parishioners did not understand Polish. The reason given by the Polish believers in their petitions was the tradition, i.e., that the Polish language had been around for a long time or that their ancestors built a particular church. The reactions of the hierarchs of the diocese of Sejny or Augustów to linguistic conflicts in the parishes depended on the number of believers of one nationality or the other in a parish. This was to maintain the principle of fairness, so that neither side would be disadvantaged. In the parishes discussed, the majority of the believers were Lithuanian parishioners, and this was taken into account in determining the proportions of sermons, hymn-singing, and other liturgical rites.The use of Polish in additional services was thus given its proper place. Keywords: Diocese of Sejny or Augustów, Language, Poland, Lithuania. [From the publication]