The paper provides the first attempt of a comparative study of the two forms of Judaizers phenomenon: the Russian Subbotnik movement and the Transylvanian Sabbatarianism. The study reveals a number of shared features in their origin, development, social makeup, religious ideas and practices as well as their relationships with the authorities. The genesis of the Transylvanian Szombatosok is directly correlated to the European Reformation, and Russian Judaizers are included in the context of the so called Protestantizing movement.