We assessed the infraspecific taxa of Coryphantha robustispina (syn. Coryphantha scheeri) by analysis of 16 morphological vegetative characters measured from 67 herbarium specimens collected throughout the range of this species within the United States, two live specimens from southeastern Arizona, five live specimens from New Mexico, six live specimens from Texas, and 279 live specimens from Pima and Santa Cruz Counties (south-central Arizona). Four spine characters exhibit strong clinal variation: central spine width, central spine form (curvature), central spine size, and lower radial spine width. Other spine characters show considerable variation but no significant clinal patterns. Central, radial, and guard spine lengths and central spine form are significantly correlated with total number of mature tubercles on the primary stem (∼ age of plant). In south-central Arizona, central spine curvature varies ontogenetically; older adults produce straighter central spines. Central, radial, and guard spine lengths, and central and radial spine widths are more correlated with length of tubercle than with age of plant. Principal component analyses indicate that morphological intergradation occurs across the east-west range of this species, indicating that neither varietal nor subspecific divisions are warranted at this time. These results raise fundamental questions regarding the legal status of Coryphantha scheeri variety robustispina, a taxon listed as endangered under the Federal Endangered Species Act.