A small monastery was founded here by and named after Saint Alto, a wandering monk, in about 750. The vita of Alto, likely written by Otloh of St. Emmeram after 1056 and ostensibly based on oral knowledge , reports that the monastery was visited by Saint Boniface, who dedicated the church. Another 11th-century text notes that Boniface also dedicated the church in nearby Benediktbeuern Abbey.
Sometime before 1000 the Welfs enlarged it and made it into a Benedictine abbey. Welf I, Duke of Bavaria resettled the monks in 1056 to the newly founded Weingarten Abbey in Altdorf , while the nuns formerly resident at Altdorf moved to Altomünster, where they lived until the monastery was dissolved in 1488 by Pope Innocent VIII.
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