In 1998 the Superintendence of Ostia began a program for the conservation, protection and maintenance of monuments with open air mosaic floors. The Termae dei Cisiarii was the first monument to be treated (1994-96). In 1998, with the introduction of a special law for the 2000 Jubilee, a conservation program was created for the Piazzale delle Corporazioni. This program included the maintenance of the floors and indirect measures of protection such as new paths for visitors. Piazzale delle Corporazioni was one of the main squares of Ostia and is still today a key transit node for visitors. The ancient monument now consists of remains of the walls and about 1900 square metres of pavement. Of these, about 1000 sq. metres are mosaic, some in situ, others detached and laid on a cement base during the 1960s. The remainder is reconstructed in cement. This paper presents the maintenance program of all the floors, carried out during June and July 1998. The techniques, time and costs of the treatment, the conception of informing the public as the work progresses and the planned measures for protection and future maintenance are described.