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Mountain lion (Puma concolor) predation caused populations of bighorn sheep (Ovis canadensis) in 2 mountain ranges in California to decline to low densities. In the Granite Mountains in the eastern Mojave Desert, lion predation reduced the sheep population to 8 ewes and held it at that level for 3 years, after which the predation abated and the population has increased at 15%/year for 3 years. Annual survivorship of radiocollared ewes was 62.5% for the first 3 years of study and ail mortalities were from lion predation. Mountain lion activity increased in the southern Sierra Nevada on winter ranges between 1976-1988. During this period, 49 sheep killed by lions were found on the winter ranges of the Mount Baxter population. Beginning in 1987, the larger subpopulation of this herd abandoned use of its low-elevation winter range. Increasing mountain lion predation is the best explanation for these habitat shifts. The Mount Baxter sheep that remained at high elevations missed a growing season on the winter range, which was reflected in lower fecal nitrogen levels. The population has declined to <20% of earlier census totals as a result. Mountain lions effectively halted a previously successful restoration program for bighorn sheep in the Sierra Nevada and reversed the overall population trend.