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The maximum turbidity zone (MTZ) of the Gironde Estuary is a site of important mineralization of particulate organic nitrogen. Moreover, this MTZ is characterized by intense cycles of settling and resuspension of anoxic fluid mud at both tidal and neap-spring time-scales. In the upper layer of the fluid mud, which was deposited at tidal slack and eroded during each ebb and flood, a denitrification rate of up to 65 mol l1 h1 was measured during the transient period of settling. High concentrations of NO2 (up to 13 M), and N2O (up to 1500 nM) were observed in this layer at tidal slack, either due to an incomplete nitrification at low oxygen concentrations or to high denitrification activity. This suggests that deposition and erosion of the upper fluid mud at the tidal time-scale could be an important source of N2O for the estuarine overlying waters. The deepest layer of the fluid mud was more turbid, NO3 depleted and remained stable throughout tidal cycles, during several days. In this layer, ammonification was responsible for an increase of NH+4. Most probable number (MPN) counts of ammonium oxidizing bacteria in turbid water and fluid mud revealed that they were closely associated with particles, whereas potential nitrification rates were rather constant (10– 14 mol l1 h1) irrespective of the suspended matter concentrations. In the MTZ of the Gironde, nitrogen behaves like in activated sludge reactors, with oxic/anoxic oscillations occurring at the tidal time scale. Nitrification takes place in th