The genus Mammillaria (Cactaceae subfamily Cactoideae) comprises around 170 species of low growing cacti possessing tuberculate stem morphologies, distributed throughout Mexico, the Caribbean, northern South America and the southern USA. Generic delimitation is difficult, morphologically due to close relationships with Neolloydia, Coryphantha, Ortegocactus and Escobaria. A number of infra-generic classifications of Mammillaria have been proposed, and although based primarily on morphological data, they are largely incongruent. Preliminary chloroplast DNA sequence data suggest that Mammillaria, as currently circumscribed is not monophyletic, and that previously proposed infra-generic classifications need to be re-evaluated. This paper presents results from chloroplast DNA sequence data aimed at assessing the generic boundaries of Mammillaria and its infra-generic classification.

Key words: Cactaceae, Mammillaria, phylogenetics