Mont. Code Ann. § 7-2-2423

The clerk or clerks of the district court of the county or counties from which the new county has been created shall, after the creation of the new county, remove from the list of jurors and jury boxes of the clerk's county or counties the names of all persons upon the list that may have been filed with the clerk by the jury commission who may appear to the clerk to be residents of the new county and certified by the clerk as provided in 7-2-2422.