AbstractCrossidium asirenseis characterized by erect-spreading, ovate-Ianceolate, mucronate leaves, partially revolute leaf margins, thin-walled filaments restricted to the costa, lamina cells with 1–2 papillae on both sides and a central strand consisting of 6–9 hydroids. Morphologically this species is related toC. aberrans(Egypt, Algeria, North America),C. apiculatumandC. spiralifolium(both South Africa). The type is from the Asir Mountains (south-west Saudi Arabia).