Roots not proliferous. Stems ascending or short-creeping, infrequently branched; scales dark brown to blackish throughout, narrowly deltate, 2--4(--5) × 0.2--0.5 mm, margins entire or shallowly denticulate to serrulate. Leaves monomorphic. Petiole dark reddish brown proximally, often fading to green distally, lustrous, 2--20 cm, 2/3--2 times length of blade; indument of black filiform scales and minute hairs. Blade deltate, 2--3-pinnate, 2.5--10 × 2--6.5 cm, thick, hairs dark, scattered, minute; base truncate; apex acute to acuminate, not rooting. Rachis greenish throughout or sometimes reddish brown proximally, lustrous, sparsely pubescent. Pinnae in 4--10 pairs, deltate to lanceolate; most proximal (largest) pinnae 1.5--4 × 1--2.5 cm; base obliquely obtuse; segment margins coarsely incised; apex acute. Veins free, evident. Sori 1--numerous pairs per pinna [1--6 pairs per segment], on both basiscopic and acroscopic sides. Spores 64 per sporangium. 2 n = 144.
Cliffs; 1675--2300 m; Ariz., Colo., Utah; Eurasia; Africa.
Asplenium adiantum-nigrum is principally a Eurasian species and occurs extremely rarely in North America (see M. G. Shivas 1969 and M. D. Windham 1983 for a discussion of the conspecificity of Western Hemisphere and Eastern Hemisphere material). It is an allotetraploid derived from hybridization of two European taxa, A . cuneifolium Viviani and A . onopteris Linnaeus (M. G. Shivas 1969). Hybrids involving A . adiantum-nigrum and other Asplenium species occur in Europe but are unknown in North America.