The Gallo-Roman site of St. Romain en Gal was discovered by chance during the building of a secondary school when houses and mosaics were uncovered. To avoid such situations in the future in consideration of city growth, an area of about 40 hectares has been surveyed and the location of archaeological features identified. Resistivity measurements have been able to give indications of the major road and of large buildings on a rectangular grid; however, some uncertainty remains on the boundaries of the site, assuming that walls or ditches did exist. A most probable area can be identified with certainty, utilizing other sources of data -- altimetry, surface finds, and ancient cultivations (1809) reported on conventional maps.