DIOCLETIANOPOLIS (Mitko Madzharov – m_madjarov@abv.bg, Dimitrinka Tancheva) Room No. 04 was a second caldarium. The stratum was 1.40 m thick, containing fragmentary bricks, mortar and Late Antique sherds. A square opening, 30 cm by 30 cm in size, was discovered on the northern wall of the caldarium, to the east of its entrance and at the floor level. The entrance towards the adjacent Room No. 05 was explored. It was 1.60 m wide, with jambs built of bricks and paved with bricks. The floor in Room No. 05 was constructed from bricks plastered with a layer of mortar over which marble slabs were arranged. The walls had marble socles. A groove, 20 cm wide, was found into the floor. It entered trough the southern wall of the room towards the caldarium and to the north it was directed towards the catchment of the spring, which supplied the thermae with warm mineral water. The groove was used to supply water for the two pools in the caldarium. The two caldaria in the thermae had separate water-conduits. A vaulted niche built of bricks was discovered, 1.22 m wide and 1.65 m high. Initially, it was an entrance. After the architectural complex was extended to the east, this entrance was closed with the wall of Room No. 02, which was a tepidarium, and a niche was designed. The extension of the thermae happened soon after their construction at the end of the 3rd century AD.