This study presents a detailed survey and discussion of the fine structure ofAmbrosiapollen. Representatives from all evolutionary groups and geographical regions are used, including 37 species and four hybrids. No significant differences are found among the species, a circumstance supportive of the congeneric nature ofAmbrosiaL. andFranseriaCav. hypothesized earlier from other evidence. The similarities of the pollen ofAmbrosiato that of the Heliantheae and Anthemideae are discussed. Newly observed structural features of the polar and colpal regions are pointed out which support an intermediate position of the Ambrosieae between the Heliantheae and Anthemideae. Possible specializations ofAmbrosiapollen within the tribe are suggested. The data also indicate that among the three closely related genera,Ambrosia, XanthiumandHymenoclea, the relationship ofAmbrosiatoHymenocleais closest.