As part of the four-year program of research by French and Italian scholars, on the derivation and distribution of water to Ostia the Ecole Française de Rome has chosen to analyze the largest castellum aquae of the city. This structure is situated nearby the Porta Romana, on the inside of the late Republican city walls. It had been previously excavated in 1985-1986, though the excavation was incomplete. Using exclusively the stratigraphy of the standing structures, it was possible to propose, at the end of this campaign, a preliminary interpretation of the remains. This interpretation will be devloped and completed after the second campaign, set to take place from June 28 - July 24, 2004. (Margherita Bedello Tata)