Crustose coralline algae occupied ∼1%-2% (occasionally up to 7%) of the sea floor within their depth range of 15-50 m, and they were the dominant encrusting organisms and macroalgae beyond 20 m depth in Young Sound, NE Greenland. In the laboratory, oxygen microelectrodes were used to measure net photosynthesis (P) versus downwelling irradiance (Ed) and season for the two dominant corallines [Phymatolithon foecundum (Kjellman) Düwel et Wegeberg 1996 and Phymatolithon tenue (Rosenvinge) Düwel et Wegeberg 1996] representing > 90% of coralline cover. Differences in P-Ed curves between the two species, the ice-covered and open-water seasons, or between specimens from 17 and 36 m depth were insignificant. The corallines were low light adapted, with compensation irradiances (Ec) averaging 0.7-1.8 μmol photons·m-2·s-1 and light adaptation (Ek) indices averaging 7-17 μmol photons·m-2·s-1. Slight photoinhibition was evident in most plants at irradiances up to 160 μmol photons·m-2·s-1. Photosynthetic capacity (Pm) was low, averaging 43-67 mmol O2·m-2 thallus·d-1 (∼250-400 g C·m-2 thallus·yr-1). Dark respiration rates averaged ∼5 mmol O2·m-2 thallus·d-1. In ice covered periods, Ed at 20 m depth averaged ∼1 μmol photons·m-2·s-1, with daily maxima of 2-3 μmol photons·m-2·s-1. During the open water season, Ed at 20 m depth averaged ∼7 μmol photons·m-2·s-1 with daily maxima of ∼30 μmol photons·m-2·s-1. Significant net primary production of corallines was apparently limited to the 2-3 months with open water, and the small contribution of corallines to primary production seems due to low Pm values, low in situ irradiance, and their relatively low abundance in Young Sound.