Calcification and primary production responses to irradiance in the temperate coralline alga Lithothamnion corallioides were measured in summer 2004 and winter 2005 in the Bay of Brest. Coralline algae were incubated in dark and clear bottles exposed to different irradiances. Net primary production reached 1.5 μmol C g−1 dry wt h−1 in August and was twice as high as in January-February. Dark respiration showed significant seasonal variations, being three-fold higher in summer. Maximum calcification varied from 0.6 μmol g−1 dry wt h−1 in summer 2004 to 0.4 μmol g−1 dry wt h−1 in winter 2005. According to P-E curves and the daily course of irradiance, estimated daily net production and calcification reached 131 μg C g−1 dry wt and 970 μg CaCO3 g−1 dry wt in summer 2004, and 36 μg C g−1 dry wt and 336 μg CaCO3 g−1 dry wt in winter 2005. The net primary production of natural L. corallioides populations in shallow waters was estimated at 10-600 g C m−2 y−1, depending on depth and algal biomass. The mean annual calcification of L. corallioides populations varied from 300 to 3000 g CaCO3 m−2. These results are similar to those reported for tropical coralline algae in terms of carbon and carbonate productivity. Therefore, L. corallioides can be considered as a key element of carbon and carbonate cycles in the shallow coastal waters where they live.