ISERNIA, a town of Italy, capital of a district in the province of Campobasso, is pleasantly situated among the Apennines, 51 miles north-east of Naples. The town, which is closely built and dirty, consists chiefly of one long narrow street running along the crest of a hill from south-west to north-east, near the middle of which are an ancient arch and a fine old marble fountain. Of the numerous Roman antiquities in and near the town the most considerable is the subterranean aqueduct, which may be traced for the distance of about a mile, and which is still used to supply the fountains and manufactories of Isernia with water. There is also a fine old Roman bridge just outside the town. On a hill half a mile distant is a chapel, once much frequented, to the saints Cosmas and Damian. Isernia has manufactures of woollens, paper, pottery, and tiles. It is the seat of a bishop, and of a civil and criminal court. Population in 1875, 9066.

Isernia is the ancient Samnite town YEsernia, which was conquered and colonized by the Romans about 264 B. C. The massive polygonal walls which form the basis of the present walls in nearly their entire circuit are attributed to the Samnites. During the social war Isernia was captured by the allied Italians, and became for a time their headquarters, and at the conclusion of the war was so severely chastized by the Romans as to be almost deserted. Its fortification in the Middle Ages seems to have been an occasion for destroying many of the Roman remains, a result which numerous earthquakes have helped to attain. That of 1805 overthrew the cathedral and did much damage. In 1799 Isernia was stormed by the French, and in 1860 it was sacked and suffered fearful atrocities during a Bourbonist insurrection.