HATTO I. (850?-913), archbishop of Mainz, belonged to a Swabian family, and was probably educated at the monastery of Reichenau, of which he became abbot in 888. The German king, Arnulf, appointed him archbishop of Mainz in 891. He presided over the synod of Tribur in 895, accompanied the king to Italy in 894 and 895, and in 899, when Arnulf died, became regent of Germany, and guardian of the young king, Louis the Child. He compelled Zwentibold, king of Lorraine, an illegitimate son of Arnulf, to recognize Louis. In 896 he secured for himself the abbey of Ellwangen and in 898 that of Lorsch. He assisted the Franconian family of the Conradines in its feud with the Baben bergs, and was accused of betraying Adalbert, count of Babenberg, to death. He retained his influence during the whole of the reign of Louis; and on the king's death in 911 helped to secure the election of Conrad, duke of Franconia, to the vacant throne. When trouble arose between Conrad and Henry, duke of Saxony, afterwards King Henry the Fowler, the attitude of Conrad was ascribed by the Saxons to the influence of Hatto, who wished to prevent Henry from securing authority in Thuringia, where the see of Mainz had extensive possessions. He was accused of com plicity in a plot to murder Duke Henry, who in return ravaged the archiepiscopal lands in Saxony and Thuringia. He died on May I S, 913. Stories of cruelty and treachery gathered round his name.
See E. Dummler, Geschichte des ost f rdnkischen Reichs (Leipzig, 1887-88) ; G. Phillips, Die grosse Synode von Tribur (Vienna, 1865) ; J. Heidemann, Hatto 1., Erzbischof von Mainz (1865) ; G. Waitz, Jahrbiicher der deutschen Geschichte unter Heinrich I. (Berlin and Leipzig, 1863) • and J. F. Bohmer, Regesta archiepiscoporum Magun tinensium, ed. C. Will (Innsbruck, 1877-86).