The Bay of Seine receives the French river with the highest nutrient concentrations, especially phosphorus. Dissolved (phosphate) and particulate phosphorus fluxes have been determined from fortnightly serial data collected by the Port Autonome de Rouen. These determinants exhibit behaviour opposite to that of the river flow: phosphate concentration decreases hyperbolically and particulate phosphorus increases linearily. An approach based on the rate of domestic effluent treatment agrees well (20%) with the measured phosphate flux. The determination of the main particulate phosphorus forms (adsorbed, apatite, non-apatite inorganic, organic) was undertaken over the whole estuarine salinity range. The speciation of particulate phosphorus exhibits a great homogeneity, irrespective of the salinity and the time of the year. From this study, it may be concluded that some 50% of the particulate phosphorus is potentially bioavailable. Heavy loads of particles expelled from the estuary during spring floods may therefore significantly contribute to the enrichment of the bay at the time when phytoplankton is actively growing.