The field season of 2011 undertaken in sector 15 of the ancient city of Apollonia, brought new and important data about the date of the semicircular structure identified during the previous years. The excavations identified the foundations of the apsidal structure, built within an earlier _insulae_ quarter. It was symmetrically located in relation to the longitudinal axis of the _insulae_. The construction of the apsidal structure lead to significant transformations of the area, indicated by the ruin of entire or partial walls (of the Classical and early Hellenistic periods), the destruction of a part of the road , as well as by the establishment of an embankment. The expansion of the excavation to the south of the sector confirmed its semicircular shape in the northern side of the structure: no traces of the continuation of the semicircular foundation wall was noticed in the south-eastern part; while to the south the straight foundation wall of the building’s façade was partly revealed. Only one row of sandy stone blocks of the foundation level was exposed. The inner part of the apse seems to have been supplied with three buttresses (extended toward south), though only two were so far revealed. Only the first course of the buttress that emerges from the top of the arch, was preserved. This is because the foundation of the rear wall of the large portico was built directly above it. Three stone blocks uncovered deeply in the foundation of the façade, were perhaps associated with the foundation of a central column of the structure. The embankment, which seems to have been constructed at the same time as the semicircular building (to strengthen the terrain around the structure), contained black-gloss pottery wares (mainly skyphoi), dated to the late 4th and the first half of the 3rd Centuries BC, and plates with small bases, which are typical for the late 3rd and the first half of the 2nd Centuries BC. Despite the broad chronology of the revealed material, the semicircular structure was certainly a construction of the Hellenistic period.