Flavonoids of Solanum species investigated prior to this study were mostly flavonol 3-0-glycosides. Evolution of the highly specialized section Androceras from its ancestors in Solanum and the entrance of certain of its species into arid environments have been accompanied by extensive elaboration of new flavonoid biosynthetic pathways. Three putatively natural species groups are both morphologically and chemically recognizable in section Androceras. Flavonoids of the most primitive alliance, series Androceras, are restricted to flavonol 3-0-glycosides, typical for Solanum. The more advanced series, ViolaceiJiorum and Pacificum, are also rich in flavonol 3-0glycosides but produce, in addition, flavones, C-glycosylflavones, 8-hydroxyflavonoids, and assorted methyl ethers. No C-glycosylflavones or 8-hydroxyflavonoids have previously been reported from Solanum. 8-Hydroxyflavonols with 4'-methylmyricetin B-rings in series ViolaceiJiorum contrast with chrysoeriol-type 8-hydroxyflavones found in series Pacficum; 8-0-glycosides are restricted to series ViolaceiJiorum. Most species of section Androceras are readily distinguished by their flavonoid profiles, including several cryptic species of previously controversial status. With 2000 some species, Solanum is among the largest of angiosperm genera and has been a traditional stumbling-block for taxonomists, partly because of its prodigious size and partly due to complex variation patterns. Recent progress has been made in delineating natural species groups or sections in Solanum (Seithe, 1962; Gilli, 1970; D'Arcy, 1972), enabling taxonomists to take a meaningful, phylogenetic approach to the genus that was not possible for earlier workers. Several natural sections of Solanum have now been subjects of detailed taxonomic study (Correll, 1962; Roe, 1971; Edmonds, 1972; Nee, in press). My own research on section Androceras (Whalen, 1977) was multidisciplinary and final conclusions on species delimitation and relationships were based on morphological, chemical, cytogenetic and phytogeographic criteria. The purpose of the present paper is to report comparative investigations of foliar flavonoid compounds in section Androceras and to discuss their bearing on the taxonomy of that group. Section Androceras includes twelve herbaceous Solanum species, mainly of Mexico and the southwestern United States. It is characterized by epidermal prickles, dissected leaves, unbranched monochasial cymes, enantiostylous flowers, dry berries, and prickly accrescent calyces that function in fruit dehiscence. Comparative morphology of section Androceras species suggests that the group consists of three natural alliances. These have been named as taxonomic series (Whalen, 1976). A synopsis of 1 L. H. Bailey Hortorium and Section of Ecology and Systematics, Cornell University, Ithaca, NY 14853.