A chemotaxonomic study of practically all the species of the genus Aloe showed that flavonoids occur as major compounds in 31 out of a total of 380 species investigated. Flavanones and dihydroflavonols are present in the exudate of species in Aloe ser. Rhodacanthae and Superpositae and also in a number of the endemic species from Madagascar. Flavones occur as the only major compound in the leaf extracts of the sects. Leptoaloe and Graminialoe. In ser. Macrifoliae and in Lomatophyllum, the sister genus of Aloe, isovitexin co-occurred with the C-glucosylanthrone aloin. The chemotaxonomic implication of these results are discussed together with the significance of the taxonomic and chemogeographical distribution of flavonoids in Aloe. With a few rare exceptions, the leaf compounds from two different biogenetic pathways (polyketide pathway and flavonoid pathway) are mutually exclusive. Since flavonoids are restricted to the basal groups in Aloe, we conclude that flavonoids are plesiomorphic characters in Aloe reflecting ancient phylogenetic and biogeographic links.