Annual differences in acorn crop size of California oaks do not correlate with rainfall the year before, but instead with weather conditions favorable for pollination and, in two species, rainfall 1 and 2 years prior to acorn fall. Despite considerable differences in mean productivity, correlated in part with local differences in water and nutrient availability, acorn production by individual trees within populations are generally synchronous. Synchrony extends over fairly large geographic areas, although whether on a statewide scale is not yet known. Knowledge of acorn production patterns may facilitate conservation of oaks rangelands, improve our understanding of wildlife ecology and provide insights into the social structure of California's Native Americans.