Hourly measu rements of pollen emission were made from cultivated plots of Ambrosia, Phleum, Zea, and Ricinus over the course of several pollination seasons as part of a study of pollen dispersion from known sources. A characteristic diurnal emission pattern was found for each genus Ambrosia pollen emission normally begins an hour or two after sunrise, peaks a few hours later, and decreases through the afternoon. Phleum starts during the night, peaks about 2 hr after sunrise, and declines slowly through the day. Zea emits pollen fairly uniformly during the period from 2 hr after sunrise to about sunset, while Ricinus pollen was collected from several hours after sunrise to late afternoon with a peak in mid-morning. Daily patterns often vary from the seasonal mean in response to changing meteorological conditions.