The production of calcium carbonate by a dense Ophiothrix fragilis population was calculated in order to investigate its role in the carbon budget of a temperate coastal ecosystem (Dover Strait, eastern English Channel). Production, calculated using monthly data of population density, demographic structure and a size/CaCO3 conversion, was 682 g CaCO3 m(-2) yr(-1). Assuming that the molar ratio of CO2 released to CaCO3 precipitated varied with temperature between 0.66 and 0.73, this production would result in the release of 4.8 mol CO2 m(-2) yr(-1). This calculation reinforces the suggestion that this coastal system is a source of CO2 to the atmosphere.