DIOCLETIANOPOLIS (Mitko Madzharov – m_madjarov@abv.bg, Dimitrinka Tancheva) Room No. 3 of the second construction period of the thermae was explored, situated in the northern end of the architectural complex. A thick layer of debris from arches and vaults was discovered. The walls were built in opus mixtum. Three vaulted niches were situated on the inner sides of each wall of the room. A mortar plaster with imprints of marble veneer was documented. The floor was covered with square bricks, 50 cm by 50 cm in size and 5 cm thick, covered with mortar plaster paved with marble tiles. Part of the channel for rainwater drainage from the roof of the thermae was explored. The finds from the excavations included fragmentary roof-tiles and bricks, Late Antique sherds, animal bones and a fragment from the upper part of a small marble column of an altar rail dated after AD 500. The stratigraphy of the site showed that the destruction of the thermae happened suddenly. It might have happened during an earthquake. The layout of Room No. 3 was identical to the layout of the neighboring Room No. 2 which was a tepidarium. The extension of the thermae and the construction of two tepidaria and one caldarium with three pools happened during AD 150 – 200.