Mycelium infecting thallus of Acarospora and transforming the areoles of the host into its own lichenized thallus. Areoles maintaining the shape of the host. Cortex yellowish green, K+ yellow. Medulla K-, I+ violet. Apothecia formed at the margin of the areoles, up to 0.4 mm in diameter, flat, black, often united into bigger, irregular apothecia. Epihymenium, exciple and hypothecium dark brown, K+ purple, hymenium hyaline, c. 70 µm high. Asci clavate, 57 -62 x 15- 18 µm, 6 (-8)-spored. Ascospores broadly ellipsoid, ends rounded, 1-septate, dark brown, 12-15 x 6.5-8.5 µm.
Rhizocarpon cleptophilum belongs to the interesting group of specialized, obligate parasites, which take over the areoles of the host lichen and transform them to their own thallus (Poelt 1990). After some time the areoles die. New areoles are not produced directly on the rocky substratum. Rhizocarpon effiguratum and R. parvum are the only species mentioned by Poelt with dark, 1-septate ascospores, yellowish green cortex and I+ reaction of medulla. The first-mentioned species is an occasional parasite on Acarospora oxytona, the other on Tremolecia atrata. Rhizocarpon cleptophilum appears to be restricted to Acarospora picea.