Three kinds of variability in the Rocky Mountain flora of Alberta are considered. The flora is composed of species with two gross distribution patterns: the boreal element and the cordilleran element. There are minor floristic differences from valley to valley; such differences involve the sporadic occurrence of certain species in different valleys. Main consideration is given to another type of floristic variability which involves a major floristic difference between the southern part of the mountain region and the northern part.The Crowsnest–Waterton area in the southern part of Alberta is distinctive climatically and phytosociologically from the rest of the Rocky Mountain region of Alberta. Moreover, a large number of plant species and some animal taxa occur in Alberta only in this area. Two possible features are suggested as being responsible for the distinctiveness of the southern area: the climatic effects of the occurrence of the area within a major storm track; and the existence of an adjacent unglaciated area which, during Pleistocene, could have served as a refugium for taxa which subsequently expanded in the southern area.