The article focuses on the textual criticism of the Ruthenian translation of the Czech book entitled Lucidář (Lucidarius), a medieval encyclopedic treatise consisting of the student’s questions and the teacher’s answers, which was most widespread in the Cyrillic manuscript tradition of the Polish-Lithuanian Commonwealth (the Grand Duchy of Lithuania and the Kingdom of Poland). This translation was made in 1636 from a non-extant edition (*Olomouc, 1622) and is represented by at least nine manuscript copies: five of them have been published and other four still remain practically unknown (kept St. Petersburg and Yaroslavl). All of them are involved in this study aiming to identify cases of a complete substitution of original (translated) texts of the teacher’s answers to some of the student’s questions with new texts. They reflect a critical approach of Ruthenian copyists to the ideas about the world set forth in Lucidarius translated from Czech. The process of replacing some of the texts went on, increasing in extend, during the 18th–early 19th centuries and affected more than half of all the manuscripts under consideration. Consequently, this Ruthenian translation of the Czech Lucidarius is to be characterized as an open textual tradition, since its content was partially (but regularly) adapted by scribes to meet their own cultural needs.