The article presents the most frequent surname in Lithuania — Kazlauskas. Referring to the article “Mysterious Lewandowski” by K. Skowronek (2000), an attempt has been made to account for this frequency in three various ways. First, the principles behind the quantitative structure of anthroponomasticons (Zipf’s law) and the loss of surnames (genetic drift) are discussed. Then the Slavic origin of the surname under consideration has been highlighted as a typical trait of the majority of surnames in Lithuania. In connection with this fact, it has been stressed that caution must be exercised in proposing a thesis on its origin as a translation from Lithuanian on a mass scale, since this thesis requires plentiful empirical evidence. Finally, the etymology of the name is analyzed. Morphologically it is a typical surname derived from a toponym. This supposition is additionally supported by the existence in Poland of numerous localities called Kozłów, Kozłowo or similar name; these in turn are most likely to have been derived from appellative-based personal names of their owners or inhabitants, such as Kozieł.