Bacterial nitrification and denitrification were studied in the sediments of the Bassin d'Arcachon, a macrotidal lagoon located in the Southwestern Atlantic French coast colonized by extensive Zostera noltii meadows. Nitrifiers were enumerated by MPN method. Densities were low : from 100 to 10,000 bacteria per ml of sediment presenting an increase during winter. Nitrifying activities were measured by ammonia-accumulation N-serve block technique. Activities were ranged from 0 to 90 µmol/m2/h and nitrification represented 120 to 140 mmol/m2/yr for intertidal sediments. The 15N isotope pairing technique was applied to sediment samples from the whole bay for determining the denitrification rates. Average rates were 10 to 30 µmol/m2/h with a maximum of 75 µmol/m2/h. Globally, coupled nitrification-denitrification was the major process during summer whereas only nitrates diffusing from the water column fed denitrification in winter. The relative importance of internal processes in the bacterial N-cycle of the bay were 3-14, 12-24 and 3-16 % of the N-nutrients inputs for nitrification, denitrification and N-fixation, respectively. The studies show that the contribution of bacterial processes in the N-cycle of the bay is low compared to external N-loadings and N-assimilation by the plants.