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We used long-term El Nino southern oscillation (ENSO), rainfall, and deer harvest records to investigate effects of ENSO and rainfall on mule deer (Odocoileus hemionus) population trends in the Sonoran Desert, southeastern California, USA. We found significant relationships between the southern oscillation index and rainfall (R-2 = 0.38, P less than or equal to0.001), and between rainfall and annual deer harvest (R-2 = 0.25, P less than or equal to 0.001). We also found that deer harvest (i.e., an index of deer abundance) in any year was related to accumulations of rainfall >5 years before that. hunting season (R-2 = 0.34), whereas the change in harvest between years (i.e., an index of rate of population change) was most related to rainfall the year immediately prior to that hunting season (R-2 = 0.15). Fluctuations in deer populations in the deserts of California ultimately may be caused by ENSO events.