Guastalla, Diocese of (GUASTELLENSIS), in the province of Reggio Emilia (Central Italy) on the left bank of the Po at its junction with the Crostolo. Until the tenth century it was an obscure hamlet, near the castle of the Marchesi di Canossa. In 998 Gregory V consecrated there the church of St. Peter (la Pieve). In 1106 Paschal II held at the same place a council of investitures. During the struggle between the popes and the Hohenstaufen the town fell under the control of Reggio; in the fourteenth century it belonged to Cremona, and later to Milan. In 1406 Filippo Maria Visconti made it a county (contea) and gave it to Guido Torelli of Mantua. Ferrante I, Gonzaga, ruled there in 1538; in 1621 it became a duchy and remained in the hands of the Gonzaga family until 1746. Later it was joined (1748) to the Duchy of Parma given to Philip Bourbon. It formed part of the Cisalpine republic in 1798, and in 1805 was given as a principality to Pauline Borghese. In 1815 the Treaty of Venice assigned it as a duchy to Marie Louise, wife of Napoleon I, and after her death, in 1847, it went to the Duke of Lucca, who in 1848 made it over to Modena. In 1860 it was joined to the Kingdom of Italy.
Ecclesiastically it formed a part of the Archdiocese of Reggio until 1471, when it became an archipresbyterate nullius. Sixtus V (1583) gave it abbatial rank; it was only in 1828 that Leo XII, at the wish of Marie Louise, made it a bishopric, with Modena as metropolitan. Its first bishop was John Neuschel, a Hungarian abbot, and chaplain to the duchess. Among his successors of note was Monsignor Pietro Rota (1855-71), afterwards translated to Mantua. The diocese has 26 parishes, 65,000 souls; 11 convents, and 2 girls’ boarding schools; it has a weekly and a monthly Catholic paper, and is the headquarters of a flourishing Catholic “Unione Agricola”.