Mont. Code Ann. § 70-16-208

Whenever any lands belonging to different persons in severalty have been enclosed and occupied in common or without a partition fence between them and one of the occupants desires to occupy that occupant's part in severalty, the other occupant shall, within 6 months after being notified in writing, build and maintain the other occupant's proportion of the partition fence as may be necessary for that purpose, and in case of neglect or refusal so to do, the person giving the notice may build the fence at the expense of the person neglecting or refusing. The amount expended may be recovered in an action, together with all damages that person may sustain on account of the neglect or refusal.