The article presents an analysis of a genealogical chronicle of the Bilevičius family from the 15th to the first half of the 17th centuries that was found in Lviv, and was written up in the beginning of the 17th century. It is an exclusive genealogical written source relating to the Grand Duchy of Lithuania, which mentions 520 individuals and presents up to five generations of the history of another 51 families (besides the Bilevičius family).