When the plaintiff does not know the name of the defendant, the defendant may be designated in any pleading or proceeding by any name. When the defendant's true name is discovered, the pleadings or proceedings may be amended accordingly.
Mont. Code Ann. § 25-5-103, under Designation of Parties.
Mont. Code Ann. § 25-5-103
When the plaintiff does not know the name of the defendant, the defendant may be designated in any pleading or proceeding by any name. When the defendant's true name is discovered, the pleadings or proceedings may be amended accordingly.