The plain of the Gharb is well known for its fertility, with well-watered soils and a dense hydrographic network. The large number of ancient towns and the frequency of rural settlements found in survey make it a key area in “useful” Marocco. There is no doubt that the Mauretanian and Roman town of Gilda is to be found in the south of ancient Marocco, somewhere along the oued Beht, an affluent of the Sebou. The name is a cognate of the term aguellid, (GLD in Libyan script) which designates the chief of a tribe or a royal prince. Two ancient sites are candidates for the identification, Rirha and the Domaine du Beht, once the Priou farm. They are roughly 7 km. apart. The nature and the size of the ancient site of Rirha in the pre-Roman periodi s uncertain. The traces uncovered are limited to structures built in mud brick and to various objects – pottery, coins, objects in ivory and ostrich shell – datable between the third century b.C. and the first century AD. The finds are evidence for close connections between the site and the network of Mediterranean exchange, and particularly avec the ‘Circle of the Straits’. Rirha in the Roman period is better documented: it consists of an agglomeration of around 10 ha. Inserted in a meander of the Oued Beht. Two domus have been excavated, one with a private bath with rooms floored with mosaics. Geophysical survey has revealed a number of masonry buildings. Finds suggest that occupation continued until the end of the third century AD, a date at which Roman finds disappear. A further occupation of the site is signalled by a radical change in the pottery and the habits of consumption in the eighth and ninth centuries AD, with the arrival of an islamised population. The new settlers reused the ruins of the Roman agglomeration. Parallel to the archaeological activities the Rirha mission is conducting a broad paleoenvironmental study that associates geomorphology, paleobotany, malacology and archeozoologie to investigate the Ancient landscape.