| Summary: | The grass genus Aristida, composed of about 330 species, is found throughout the tropical and warm-temperate regions of the world. In México, about 50 species are present, mainly in the arid to subtropical zones. Northeastern México, which includes the states of Coahuila, Nuevo León and Tamaulipas, is considered a center of diversification for the species of the arid and semiarid zones. The taxonomy of Aristida for this area is revised where the genus is represented by 12 species and in total 23 infrageneric taxa. The morphological diversity of the genus in northeastern México is attested to by the disposition of its species in five informal groups, by the division of several of the species into varieties or forms, and by the occurrence of single- and three-awned populations and contracted- and spreading- panicle populations within the same species or group. Identification keys, selected synonymy, distribution maps and collection records are provided.
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