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Pollen and macrofossil analyses of a radiocarbon-dated core, 1125 cm long, from Montezuma Well, elev. 1125 m, Yavapai County, Arizona, provide a record of summer precipitation similar to that of other monsoonal regions of the world. Precipitation was least from 4000 to 5000 yr B.P., and generally was greater than today before 8400 yr B.P. During the early-Holocene moist period, oak and grass pollen are abundant, and the best modern analogs are within the Arizona Monsoon boundary, so increased summer precipitation is indicated. Climatic parameters are estimated with the technique of best modern analogs for fossil samples. Temperature was highest ca. 3400 and 5700 yr B.P., and generally was cooler than today before 6800 yr B.P. Brief cold intervals in the Holocene match periods of global cooling and of positive C-14 and Be-10 anomalies.