A research excavation was commenced on the site of the basilica which forms the western side of the forum of Calleva Atrebatum (Silchester, Hants) in IQ80. Its aim is to examine the Late Iron Age and early Roman occupation which, despite extensive Victorian excavations, was preserved beneath the masonry basilica. So far there is evidence of the Iron Age sequence dating back to the last quarter of the first century B.C.; it ends with the construction of a palisade dating to about the time of the Roman conquest. Two major phases of Roman timber building have been recorded, of which the later consists of a large basilica, interpreted as part of a forum-basilica and of Flavian date. The masonry basilica dates to the early second century. From the mid third until the later fourth century the basilica was given over to metalworking. The amphitheatre, with its well-preserved earthen seating banks, was first constructed during the third quarter of the first century A.D., when the seating, arena wall and entrance passages were built of timber. After several phases of repair the arena wall and entrance passages were rebuilt in stone in the first half of the third century. The full plan of this phase has been recovered; it consists of two opposing entrances on the long axis and two apsidal recesses on the short axis. The monument enjoyed a brief period of reuse in the twelfth and early thirteenth centuries.