Monte Miseno is one of the tens of volcanoes forming the Phlegrean Fields near Naples. Its northern slope is marked at the base by a small terrace that rests at 7-8 meters a.s.l. and consists of alternating beach deposits, sometimes fossiliferous, and fluvial-palustrine sediments. This sequence occurs also within the ruins of a Roman thermal building that was built up during the 2nd century A. D. and then abandoned toward the end of 4th century. Two stratigraphic sections located within the ruins were studied in detail in order to clearify the sedimentary and environmental evolution. The archaeological content of some layers and the analysis of the relationships between sedimentation and episodes of late re-occupation of the building (into which furnaces for ceramics had been active during 6-7th and 11-12th centuries) allowed us to date some of the events recorded in the sequence. The data obtained permit to give new interesting details about timing and rating of the bradiseismic movements that affected the area during mediaeval times. The data gained at Miseno's Terma, coupled with those regarding the submerged ruins of the roman harbour of Miseno, allow to estimate a total subsidence of 10-11 meters between the 8th and 10th century, and a total uplift of some 5 meters between the 13th century and today. Another important result of our study is that the mediaeval bradiseismic oscillations affected a much wider area of the Phlegrean Fields than the one indicated by previous Authors, who assumed that it had the same extent and position of the area of modern bradiseisms.