Jean-Francois Fortin, the only new Bloc MP to be elected in the May election, declared his candidacy Saturday at a party meeting in Drummondville, Que.
Bloc MP Maria Mourani, another one of the party's remaining MPs, is also expected to run.
She had been pushing for the race to be delayed until the New Year but the party decided today to go ahead sooner than that.
Party members will hold a vote Dec. 11 after a fall leadership race.
Duceppe stepped down as the longtime leader after a disastrous election that saw the Bloc cut down to four seats.