First Old Believer settlements in Poland were founded in the 17th century, after the schism in the Russian Orthodox Church. The migrants preserved their religion, language (Russian dialect from Pskov-Novgorod-Velikiye Luki region) and culture through ages in foreign surroundings. Simultaneously they achieved Polish language, but up till the beginning of the 20th century and the revival of Polish statehood, not all of them were biligual. It was the time of creating the state of diglossia in the Old Believers' language situation, i.e. hierarchical relation between the languages being used by them, with certain domains of for each language. After the Second World War serious language changes were underway, and the dialect was under strong influence of Polish language. There are all kinds of interference in the Old Believers' dialect, especially lexical, phonetic and syntactic. Contemporaneously we can observe the phenomenon of code-switching in their speech. Although scholar's interest to code-switching is as old as the theory of language contact, the universal methodological approach has not been discovered yet. This article is basing on P. Muysken's typology, differentiating insertional code-switching, alternational code-switching and congruent lexicalization. The examples of each type are analysed and compared with other typologies. There are a lot of doubtful cases of code-switching in bilingualism of the group of our interest, which do not suit precisely any of the types or can be classified as more than one of them, what is typical for structurally related systems. However, the questionable situations can not be omitted in the analysis, while they constitute inseparable phenomena of the speech of Polish Old Believers.