Origin and Habitat: This species is endemic to Argentina (South of La Rioja, East of San Juan and west of Cordoba)
Altitude: It grows at elevations of 450 to 1350 metres above sea level.
Habitat: It is found and very common on rocky patches, gravelly slopes, granite or on red sandstone mountains in scarce and almost deprived of nutrients soil, with summer daily temperatures raising above 40° C. The species grows between dense, spiny bushes in chaco forest together with Gymnocalycium saglionisSN|21407]]SN|12116]], Gymnocalycium schickendantziiSN|22430]]SN|22430]], Gymnocalycium riojenseSN|12116]]SN|21407]], Lobivia aureaSN|10646]]SN|10646]], Echinopsis leucanthaSN|8246]]SN|8246]], Parodia microspermaSN|11718]]SN|11718]], Tephrocactus inermis, Stetsonia coryneSN|19128]]SN|19128]], and Opuntia salmianaSN|4031]]SN|4031]]. There are no known major threats affecting this species.

• Gymnocalycium castellanosii Backeb.

• Gymnocalycium castellanosii Backeb.
• Gymnocalycium castellanosii subs. acorrugatum (J.G.Lamb.) R.Kiesling & D.Metzing
• Gymnocalycium acorrugatum J.G.Lamb.
• Gymnocalycium ferox (Backeb.) Slaba
• Gymnocalycium hybopleurum var. ferox Backeb.

• Gymnocalycium castellanosii subs. bozsingianum (Schütz) Amerh. & H.Till
• Gymnocalycium bozsingianum Schütz
• Gymnocalycium castellanosii var. bozsingianum (Schütz) Piltz

• Gymnocalycium castellanosii subs. ferocius (Backeb. ex H.Till & Amerh.) G.J.Charles
• Gymnocalycium ferox var. ferocior (Backeb.) Slaba
• Gymnocalycium hybopleurum var. ferocius Backeb.
• Gymnocalycium mostii subs. ferocius Backeb. ex H.Till & Amerh.

Description: Solitary plant with fairly heavy spines bent backward in a distinctive fashion.
Stem: Up to 15 cm in diameter an 10 cm tall, dull green or bluish-green, apex depressed with confluent areoles forming a woolly spineless disk.
Ribs: 10-12, with rounded sharply separated tubercles in young specimens that progressively becomes almost flat in mature plant.
Spines: Subulate, stout, thick and straight or slightly curving, all whitish, greyish or brownish, with a darker tip, up to 2,5 cm long.
Radial spines: 5-7(-8).
Central spines: 0-2.
Flowers: Bell-shaped or funnelform, up to 4,5 cm in diameter, white to very light pink with a pink or reddish throat.
Fruits: Spherical or slightly ovoidal green.

Notes: The taxonomy of this species is not clear. The former synonym Gymnocalycium feroxSN|11489]]SN|11489]] (Backeberg) Slaba 1984 seems to be incorrect.

Bibliography: Major references and further lectures
1) Edward Anderson “The Cactus family” Timber Press, Incorporated, 2001
2) David R Hunt; Nigel P Taylor; Graham Charles; International Cactaceae Systematics Group. "The New Cactus Lexicon" dh books, 2006
3) James Cullen, Sabina G. Knees, H. Suzanne Cubey "The European Garden Flora Flowering Plants: A Manual for the Identification of Plants Cultivated in Europe, Both Out-of-Doors and Under Glass" Cambridge University Press, 11/Aug/2011
4) Demaio, P., Perea, M. & Trevisson, M. 2013. Gymnocalycium castellanosii. In: IUCN 2013. "IUCN Red List of Threatened Species." Version 2013.2. <www.iucnredlist.org>. Downloaded on 03 January 2014.

Cultivation and Propagation: Summer-growing and pretty easy, water regularly in summer, and keep rather dry in winter. However it can handle excessive water to little water. It grows well in full sun or half sun, too (but in full sun it will form stronger spines).
Propagation: Seeds (it seldom produces offsets). Seed Collecting: Permit fruit to ripen. Fruit must be significantly overripe before harvesting seed; clean and dry seeds