Plants unbranched, usually deep-seated in substrate and inconspicuous. Roots diffuse, upper portion not enlarged. Stems depressed-spheric to short cylindric, (1-)2-3.5 × (1.4-)2-4(-7) cm, firm; tubercles 3-6(-8) × 2-3 mm; axils without evident hairs; cortex and pith not mucilaginous; latex clear or slightly milky, sporadic, only in outer cortex. Spines (26-)40-60(-90) per areole, in several series but all equally thin, mostly appressed, white or very pale pink, often minutely tipped pinkish brown, innocuous, bristlelike, 0.6-5(-6) × 0.05-0.1 mm, glabrous to plumose, all interpreted as radial, innermost spines shortest; central spines 0. Flowers 0.9-1.5(-2) × 0.8-1.3(-1.8) cm; outermost tepal margins entire (or minutely and irregularly lacerate); inner tepals white or cream, usually with sharply defined midstripes of green, yellow, tan, pink, pale purple, or reddish, 4.5-8 × 1.5-2.7 mm; stigma lobes yellow or pale yellow-green to green, 0.3-1 mm. Fruits scarlet, cylindric or clavate, 10-20(-25) × (3-)4-8(-11) mm, juicy mostly in fruit walls; floral remnant persistent. Seeds black, 1-1.2[-1.4] × 0.8[-1.1] × 0.8 mm, pitted; testa hard; interstices equaling pit diameters; pits bowl-shaped. 2n = 22. Flowering [Jan-]Feb-Mar; fruiting Jun-Aug. Chihuahuan desert scrub with Agave lechuguilla, rocky hills, gravelly slopes, usually on limestone; 500-1800(-2100) m; N.Mex., Tex.; Mexico (Chihuahua, Coahuila, Durango, Nuevo León, San Luis Potosí, Sonora, Zacatecas). Adults of Mammillaria lasiacantha usually have glabrous spines, but in some populations all plants may retain plumose spines at maturity. Epithelantha species and immature plants of Coryphantha vivipara var. neomexicana often are misidentified as adults of M. lasiacantha, especially from El Paso, Texas, westward (where M. lasiacantha is rare). Mammillaria lasiacantha is remarkable for its disjunction to one site in Sonora, Mexico, far to the west of its usual range.