We analyzed long-term population data for bighorn sheep (Ovis canadensis nelsoni) occupying the San Gabriel Mountains in southern California. From 1967–1972 an estimated 500 bighorn sheep occupied that range. During 1968–1975 wildfires burned over half the occupied winter–spring ranges. Wildfires increased carrying capacity, and the population grew to 665 animals in 1976. During 1976–1982 adult mortality was low and recruitment was dependent on annual forage production and weather conditions during lambing. Rate of change indicated the population would double every 44 years, and it was considered to be stable. As vegetation matured and carrying capacity declined, mortality of adults and lambs increased and the population halved approximately every 8 years to 501±30 bighorn sheep in 1989. During 1989–1995 adult mortality increased and rate of change indicated the population would halve approximately every 2.8 years. From 1995–2002 the population was stable at 90 animals. Increased recruitment and an inverse relationship between number of adult ewes and recruitment rates between 1985 and 2001 suggested that neither habitat suitability nor disease was causing the dramatic population decline observed after 1989. We hypothesize that because of a declining mule deer (Odocoileus hemionus californicus) population, mountain lions (Puma concolor cougar) began to prey more frequently on bighorn sheep, which led to a dramatic decline in the sheep population after 1989.