Resource managers have become increasingly concerned over the apparent decline of Quaking Aspen in the western United States. Factors leading to these changes include fire suppression, livestock grazing, wild ungulate browsing, conifer succession, and perhaps climate change. To aid in the development of conservation and restoration strategies for aspen, we investigated genetic variability, clonal diversity, levels of differentiation, and patterns of geographic variation among 663 aspen individuals located in 82 stands from 8 watersheds throughout the western slope of the central Sierra Nevada, California. Genetic data show that, as a group, the aspens are genetically variable, as determined by starch gel electrophoresis. Individual stands, however, consist of only one to eleven genetic individuals and are much less variable. Forty of the 82 stands are monoclonal. Watersheds are not monoclonal but instead contain between three to 41 clones (average = 25.5). The geographic patterning of genetic variation detected by the allozyme data can be used to delineate genetic restoration units and to prioritize conservation efforts. Results from the study can be used to identify priority stands for in situ and ex situ protection, including those that contain large amounts of genetic variation, are highly differentiated, or contain rare alleles.