Mont. Code Ann. § 7-3-4217

Every person who has been declared elected mayor or council member shall, within 10 days after the declaration, take and file with the city clerk an oath of office in the form and manner provided by law and shall execute and give sufficient bond to the municipal corporation in the sum of $10,000, conditioned for the faithful performance of the duties of the office. This bond must be approved by the judge of the district court of the county in which the city is situated and must be filed with the clerk and recorder of the county in which the city is situated.