A new technique has been employed to trace pollen dispersal in populations ofDelphinium cardinale(Ranunculaceae), a herbaceous perennial native to southern California. Radioactive iodine (I131) was applied directly to fresh pollen on stamens of living plants and subsequently traced and recorded on other plants in the field. While absolute amounts of I131or pollen dispersed were not determined, monitor detection showed that radioactive pollen was dispersed to open flowers (more often to the brightest flowers higher on the plants), and not randomly to stems or unopened flower buds. All dispersal was attributed to hummingbirds—the only observed flower visitors and pollen vectors for these plants. One very denseDelphiniumpopulation had almost 140 lambda of I131applied over 8 days to 76 flowers on six plants in the center of an area 22 m in diam. One fourth of this area monitored after 30 hr showed radioactive pollen on flowers of 8.9% of the 271 flowering stems; 8 days after the first application of I131another fourth of the area showed radioactivity on flowers of 58.2% of the 323 stems in it. A second study area 22 m in diam had 18 lambda of I131applied to 18 flowers on one plant in the center; 64.2% of all (137) flowering stems in the area had radioactive pollen detectable 24 hr later. It is clear that pollen from very few flowers and plants can quickly reach a large proportion of the individuals in such populations. The strongly protandrous nature of the flowers, the frequent flower visits of nectar‐gathering hummingbirds, and the regular adhering of pollen to birds' chins suggest that cross pollination (xenogamy) regularly occurs in these delphiniums. Further study of such pollen vectors' habits and flight patterns, while using pollen tagged with I131, should lead to quantitative expressions of pollen (and gene) dispersal within and among spatially separated plant populations such as these.