DIOCLETIANOPOLIS (Mitko Madzharov – m_madjarov@abv.bg, Dimitrinka Tancheva) Two rooms of the first construction period were explored in the northwestern part of the thermae. The northern half of the pool in Caldarium No. 1 was explored. Its bottom was paved with large marble slabs, the largest one 2.65 m by 0.87 m in size and 3 cm thick. Two siphons for draining the water in the drain were discovered in the southwestern corner of the pool. There was a staircase with three steps faced with marble veneer on the northern side of the pool, identical to the staircase on its southern side. Room No. 05 (Caldarium No. 1a) was situated next to the catchment of the mineral spring. Its walls were preserved up to 4.50 m in height. There were marble plinths at the bottom of the walls that separated the floor pavement from the wall facing. A large pool was situated in Caldarium No. 1a. Its bottom was paved with marble slabs placed over pavement of bricks and mortar, and its walls were faced with marble veneer. A siphon towards the drain, which ran beneath the pool in Caldarium No. 1, was discovered in the southern part of the pool, close to its staircase. A marble altar was discovered in situ on the northern side of the pool in Caldarium No. 1a. The altar had a spout of the water-conduit for filling the pool with mineral water. The finds from the excavations included fragmentary roof-tiles, fragments from marble veneer and floor slabs, several fragments from stone mortars and sherds of the 3rd – 5th centuries AD. Special attention deserved the sherds of the 8th – 9th centuries AD found on the bottom of the pool in Caldarium No. 1, indicating that it was probably used again in the Early Middle Ages.