EPA is approving a State Implementation Plan (SIP) revision submitted by the Governor of Montana on April 30, 2001. The April 30, 2001 submittal revises the State's Administrative Rules of Montana (ARM) by adding a Credible Evidence Rule. The intended effect of this action is to make the Credible Evidence Rule Federally enforceable. Finally, the Governor's April 30, 2001 submittal contains other SIP revisions which have been addressed separately. This action is being taken under section 110 of the Clean Air Act (CAA).