The area of two clones of Populus tremuloides Michaux in the Fish Lake Basin, Sevier County, Utah, was determined using aerial photographs combined with ground delineation using leaf, bark, and stem characteristics. The clones occupied 24.9 acres (10.1 ha) and 106.8 acres (43.3 ha) and contained about 15 000 and 47 000 ramets, respectively. Other clones up to an estimated 200 acres (81.0 ha) were observed in the vicinity and in other areas of the central and southern Rocky Mountains. In contrast, the largest P. tremuloides clone in eastern North America whose boundary has been verified was 3.8 acres (1.5 ha). Populus tremuloides and P. grandidentata Michaux clones of eastern North America are generally small, typically less than 0.1 acres (0.04 ha) in size. The large size of many western clones is probably due to the establishment of few seedlings, periodic fires promoting suckering, expansion over a long time period, and little competition with conifers or other vegetation. Small size of most eastern North American clones and lack of large clones are probably due to establishment of many seedlings per unit area, competition among clones and with other more shade-tolerant vegetation, and their relatively young age.