The report estimates the annual emission of the five principal air pollutants (particulates, SO2, carbon monoxide, hydrocarbons, and NO2) in the State of Montana, as of 1970. The data were obtained from voluntary responses to questionnaires sent out in behalf of the state, supplemented by published references on fuel usage and area emissions, and by personal communications. Point source losses are responsible for nearly all of the SO2 emitted in the State. Principal sources of SO2 emissions are nonferrous metal smelters followed by oil refineries and utilities; the balance is accounted for mostly by the industrial combustion of fuel. The same group of point sources accounts for about one-third of particulate emissions. Additional substantial contributers are forestry and agricultural operations, including teepee burners and slash burning, contributing about one-third, and forest wildfires and stubble burning, estimated to contribute about one-fourth.