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Lycium (Solanaceae) is a genus of similar to75 species found worldwide inhabiting and to semi-arid environments. Phylogenetic relationships were inferred for 25 species of Lycium and three closely-related Grabowskia species using sequences of the internal transcribed spacer (nr-ITS) regions of nuclear ribosomal DNA and 27 morphological characters. The nr-ITS and morphological data sets were congruent and a combined analysis showed strong support for a clade containing several North American species that have distinctive floral and fruit morphologies, In addition, there was strong support for a single origin of gender dimorphism among North American Lycium species. Inclusion of a dimorphic species from South Africa suggests that gender dimorphism has evolved independently among African Lycium. Results strongly suggest that Lycium is not monophyletic, but includes the genus Grabowskia. Further, North American Lycium are paraphyletic and current sectional circumscriptions for the American species are inadequate.