A flora from Tertiary fossiliferous shales of the upper part of the Ruby Basin in southwestern Montana is described. The intermontane lacustrine deposits consist of two phases with distinct floral assemblages. The composite flora comprises aquatic, riparian temperate deciduous- and coniferous-forest, xeric-woodland, and desert-scrub associations. The flora consists of 37 families, 61 genera, and 82 identifiable species of which 25 are heretofore undescribed. Three additional new species are described among the Incertae sedis. Approximately 12 per cent of the species are gymnosperms, and 85 per cent are angiosperms. Arcto-Tertiary and Madro-Tertiary elements are equally represented. Comparison with the Bridge Creek and Green River floras discloses a 13 per cent and 4.5 per cent respective overlap. Significantly, a 40 per cent specific identity exists between the Ruby and Florissant floras. Nearly identical floristic units suggest that the Ruby and Florissant floral assemblages were part of a single botanical province. The fossiliferous shales of the Ruby area are late Oligocene and nearly contemporaneous with the Bridge Creek shales but younger than those of the Florissant area. The Recent flora is chiefly xeric-semidesert, xeric-coniferous, Hudsonian-deciduous, and Hudsonian-coniferous.