Stem tubercles 6-21 mm. Radial spines usually 16-30 per areole. Flowers 2.2-5.1 cm diam. Fruits 6-15 mm diam. Flowering summer; fruiting fall. Semidesert grasslands, Madrean pine-oak woodlands, steep, rocky slopes, canyons, and valleys, usually on alluvial or igneous substrates; 1100-2000 m; Ariz., N.Mex.; Mexico (Chihuahua, Sonora). In rocky habitats near the Mexican border, populations of Mammillaria wrightii var. wilcoxii tend to have more radial spines and smaller flowers than elsewhere and have been segregated as M. meridiorosei. Almost half of the published descriptions and illustrations purporting to represent variety wilcoxii are misidentifications of Mammillaria viridiflora.