The southernmost range of the Humboldt Flying Squirrel, Glaucomys oregonensis, ends at the San Bernardino and San Jacinto Mountains, where isolated endemic populations occur. In 1933 Joseph Grinnell, UC Berkeley, documented their occurrence in both mountain ranges. Today, flying squirrels exist in healthy numbers in the San Bernardino Mountains; however, the last time they were observed in the San Jacintos was in 1994 by CDF&W biologist Kevin Brennan. Thus, my project was to affirm their absence and, if absent, investigate why they no longer occur in the San Jacinto Mountains. Two hypotheses pervade: (1) there has been a change in habitat, and (2) disease extirpated the population. My project addressed the first hypothesis. Therefore, my initial objective was to delineate optimal habitat parameters for flying squirrels within the occupies portion of their range, the San Bernardino Mountains. The second objective was to compare those parameters to the forests of the San Jacinto Mountains. A third objective was then to investigate their presence or absence in the San Jacinto Mountains. The long-term conservation goal is to restore biodiversity in the San Jacinto Mountains by maintaining viable populations of flying squirrels, a keystone species.