Mont. Code Ann. § 46-23-217

A prisoner who commits a crime while imprisoned in a state prison or while released on parole or under the supervised release program and who is convicted and sentenced for the crime shall serve the sentence consecutively with the remainder of the original sentence. However, the prisoner remains eligible for parole consideration under 46-23-201 in regard to the original sentence. If paroled from the original sentence, the prisoner shall begin serving the subsequent sentence.