Material from a marine littoral fringe population of Prasiola stipitata exhibited inorganic C saturation of photosynthesis when emersed but hydrated in air (35 Pa CO2) and when submersed in sea-water (2 mol m-3 inorganic C, pH 8.0). This high affinity for inorganic C was accompanied by a low CO2 compensation concentration and the capacity to use external HCO3 −. No CAM-like behaviour was exhibited in either emersed or submersed specimens. The δ13C of the alga was -21-18%. These characteristics resemble those of several other high-intertidal algae and of a high-intertidal cyanobacterial lichen which have been tested. The inorganic C metabolism processes examined do not distinguish between the Ulvophyceae and the Pleurastrophyceae as the most appropriate location for Prasiola, although chlorophyll chlorophyll b ratios (including measurements on three pleurastrophycean supratidal lichens) and polyol occurrence favour the Pleurastrophyceae as the higher taxon to which the Prasiolales should be referred.