Proc. Amer. Acad. Arts 3: 262. 1856.
Plants branched or unbranched; branches 0–9 (–17). Roots diffuse; upper portion not enlarged. Stems spheric to cylindric, usually (4–) 5–16 (–30) × (2.3–) 3.5–6.8 cm, firm; tubercles 4.8–12 (–15) × 3.5–7 mm; axils appearing naked; cortex and pith not mucilaginous; latex absent. Spines (19–) 26–33 (–38) per areole, glabrous; radial spines 17–35 per areole whitish or pale tan, bristlelike, 6–12 × 0.1–0.15 mm, stiff; central spines (2–) 3–4 per areole, 1–3 (–4) spines at least hooked (uncommonly 0 hooked), reddish to purplish brown to almost black (rarely golden brown), abaxial 1 porrect, others inconspicuous, appressed against radial spines, hookless porrect spines (3–) 9.5–25 × 0.1–0.5 mm; subcentral spines 1–3 per areole, adaxial to central spines, sometimes transitional to central spines, usually straight and barely distinguishable from radial spines. Flowers ca. 2 × 1.8–3.5 (–4.5) cm; outermost tepal margins minutely fringed; inner tepals bright-rose-pink or rose-purple, 10–16 × 4–8 mm; stigma lobes yellow-green to green, 3–7 mm. Fruits green, turning bright red, scarlet, or carmine (rarely yellowish), elongating until clavate after color change to red complete, 12–29 × 5–8 mm, juicy only in fruit walls; floral remnant persistent. Seeds black, 0.8–1 × 0.7–0.9 mm, pitted; testa hard, brittle; anticlinal cell-walls straight; interstices conspicuously wider than pit diameters; pits bowl-shaped. 2n = 22.
Phenology: Flowering Apr–Sep; fruiting Sep–Mar.
Habitat: Chihuahuan and Sonoran desert scrub, grasslands, interior chaparral, oak woodlands, alluvial slopes, hills, canyons, silty, sandy, gravelly, or rocky soils of igneous or calcareous origin
Elevation: 80-1400 m
Ariz., Calif., N.Mex., Tex., Mexico (Chihuahua), Mexico (Sonora)
Mammillaria grahamii is geographically variable. Past attempts to distinguish larger or western individuals as M. microcarpa have proven arbitrary.
All spine hooks on a plant may be oriented in the same direction, a trait sometimes mistakenly said to be limited to Mammillaria mainiae. Plants with short, straight central spines (rarely a mixture of both hooked and straight spines on the same stem) occur in some populations; they have been called M. oliviae or M. grahamii var. oliviae. The name Mammillaria microcarpa Engelmann has been widely used but was not validly published until after M. grahamii.