This article attempts to answer a simple but neglected question: what concrete steps were taken to identify and list Jews in France during the World War II? Who was in charge of the operation? What criteria and means did they use? This study focuses on the local case of Lens in northern France to determine the respective roles of “self-declaration” and “detection” to take the administrative census of Jews between 1940 and 1944. The process depended first of the self-declarations, yet the absence of declaration was no protection from discriminations, persecutions and deportations. This article also uses the letters addressed to the public authorities to analyze the wide range of manners to declare one’s Jewishness, which differed markedly from the statistical output made at the time and since.