A unique inventory of Lithuanian Metrica books compiled in September–October, 1641, which had thus far escaped the attention of Lithuanian Metrica researchers, was discovered in the Vilnius Chapter’s collections (F 43) at the Wroblewski Library of the Lithuanian Academy of Sciences. It was compiled by the notary public Fr Laurynas Mocarskis (Wawrzyniec Mocarski). Due to objective shortcomings, Mocarski’s list cannot be considered a proper inventory of the Lithuanian Metrica books, but, lacking other material, it is nonetheless an important and useful auxiliary source helping us to understand the development of the entire complex of books of the Lithuanian Metrica and subtleties in the history of individual books.