A multivariate analysis was performed for populations of Echinocereus (section Triglochidiatus) to facilitate the taxonomic circumscription of E. arizonicus subsp. arizonicus. Twenty-one morphological characters for 16 populations evidenced the validity of at least two subspecific taxa within E. arizonicus: E. arizonicus subsp. arizonicus and E. arizonicus subsp. nigrihorridispinus. Principle components analysis indicated that stem characters were most diagnostic in defining two distinct groups of populations, each including the type locality of one of the two subspecies. Unweighted pair group method with arithmetic mean (UPGMA) clustered populations of the two subspecies apart from one another and from those of the outgroup, E. triglochidiatus subsp. mojavensis. For most measured characters, means differed significantly between the two subspecies. Discriminant analysis correctly classified 97.0% for individuals of E. arizonicus subsp. arizonicus and 94.7% for individuals of E. arizonicus subsp. nigrihorridispinus, compared to an overall 97.8% correct classification of individuals for all perfect-flowered taxa of section Triglochidiatus investigated.