Joselewicz initially served in the militia before petitioning Kościuszko for permission to form an all-Jewish unit. On September 17, 1794, Kościuszko officially announced the creation of the unit. Joselewicz, along with another Jew named Józef Aronowicz, issued a patriotic call-to-arms in Yiddish denouncing Russia and Prussia, eliciting hundreds of volunteers, mostly poor trade workers and artisans. Five hundred men were eventually accepted and formed into a . At Joselewicz's request, they were allowed to keep their religious customs, including access to kosher foods, abstaining from combat on the Sabbath when possible, and growing their beards. Joselewicz's unit was popularly known as "the Beardlings". They took part in the Battle of Praga, in which the unit was wiped out, with only a few men (including Joselewicz) surviving the battle. Joselewicz himself was taken prisoner by the Russians.