The southern African species of Leonotis (Pers.) R. Br. are revised. Nine taxa are recognised including a new species and two new subspecies. A multidisciplinary approach was adopted involving: floral, vegetative, seedling and fruit morphology, karyology, palynology, pollination biology, phytogeography, phytochemistry and cladistics. Of these characters the most important in delimiting species is leaf morphology. Phytochemistry revealed qualitative and quantitative differences in essential oils between taxa. Germacrene is the dominant essential oil in seven species, particularly in L. dubia E. Mey. and L. nepetifolia (L.) R. Br. Caryophyllene dominates in L. randii S. Moore. Although certain species have characteristic aromas, no inter-specific differences are apparent in the levels of α-copaene, the dominant volatile essential oil extracted with Tenax. Essential oil data is in partial agreement with the phylogeny and species concepts presented in this document. Omithophily accounts for most fruit set in perennial species but in the annual species (L. nepetifolia) autogamy prevails. Neither omithophily nor entomophily improved nutlet vigour which was gauged from nutlet germination. Leonotis species are predominantly pollinated by a variety of sunbirds, although bees are also involved. Nectar is sucrose-dominant in perennial species but hexose-dominant in the annual. The majority of species are concentrated along the eastern seaboard with the eastern Cape forming the centre of diversity. The widespread distribution and tropical affinities of L. leonurus (L.) R. Br., suggest an origin for the genus in tropical Africa. Migration to southern Africa and subsequent speciation are suggested. Habitat diversity and edaphic conditions probably played an important role in the evolution of narrowly distributed xerophytic species. A cladistic analysis re-emphasised that Leonotis is a monophyletic group of closely related species. The lack of floral divergence in perennial species indicates the constraints imposed by ornithophily. The annual, L. nepetifolia, is highly derived and displays numerous autapomorphies. The morphology, distribution and tropical affinities of L. leonurus are interpreted as pleisiomorphic. The proposed phylogeny is reasonably compatible with phytogeographic data. / Thesis (Ph.D.)-University of Natal, Pietermaritzburg, 1995.