The present report is a result of the study of the relation between climate and forest fires in Montana and northern Idaho. This region is designated as District I of the United States Forest Service. The data used are the weather records of the United States Weather Bureau for the regular and cooperative stations, and the detail fire reports of the United States Forest Service for the years 1909 to 1919, inclusive. Mr. C. C. Delavan has compiled the fire records and Mr. J. A. Larsen the weather data.