Asplenium unisorum, in the fern family Aspleniaceae, grows from a slender, erect rhizome (underground stem), 0.5 to 3 cm (0.2 to 1.2 in) tall and 0.5 to 1 cm (0.2 to 0.4 in) in diameter, which is covered with the bases of the leaf stalks and a few small black scales. Stalks of the fronds are black and shiny, and about 2 to 5 cm (0.8 to 2 in) long. The fronds are linear, 8 to 30 cm (3 to 12 in) tall by 0.5 to 3 cm (0.2 to 1.2 in) broad, with 20 to 35 pinnae (leaflets) per side, and gradually narrowing towards the apex. The pinnae are usually strongly asymmetrical in outline, unequally triangular, with mostly entire (smooth) margins. There usually is a single marginal sorus (the spore producing body) running along the upper margin of the underside of the pinna.