Uroleucon is one of very few aphid genera that have species indigenous to South America. A new species of aphid from Chile is described and illustrated, Uroleucon eumadiae sp. nov., living on the two South American species of the genus Madia (Asteraceae): M. sativa Mol. and M. chilensis (Nutt.) Reiche. Morphological characters are discussed and the characters by which this aphid species may be distinguished from related ones are given. The long, hairy last rostral segment of the new species suggests strongly a feeding adaptation, and parallels that of a North American Madia-feeding species in the same genus which is not, however, closely related.