The Environmental Protection Agency (EPA) is finalizing revisions pursuant to section 110 of the Clean Air Act (CAA) to the Federal Implementation Plan (FIP) addressing regional haze in the State of Montana. The EPA promulgated a FIP on September 18, 2012, in response to the State's decision in 2006 to not submit a regional haze State Implementation Plan (SIP). We proposed revisions to that FIP on April 14, 2017, and are now finalizing those revisions. Specifically, the EPA is finalizing revisions to the FIP's requirement for best available retrofit technology (BART) for the Trident cement kiln owned and operated by Oldcastle Materials Cement Holdings, Inc. (Oldcastle), located in Three Forks, Montana. In response to a request from Oldcastle, and in light of new information that was not available at the time we originally promulgated the FIP, we are revising the nitrogen oxides (NO<INF>X</INF>) emission limit for the Trident cement kiln. We are also correcting errors we made in our FIP regarding the reasonable progress determination for the Blaine County #1 Compressor Station and the instructions for compliance determinations for particulate matter (PM) BART emission limits at electrical generating units (EGUs) and cement kilns. This action does not address the U.S. Court of Appeals for the Ninth Circuit's June 9, 2015 vacatur and remand of portions of the FIP regarding the Colstrip and Corette power plants; we plan to address the court's remand in a separate action.