CASTELNAU, MICHEL DE, SIEUR DE LA MAUVISSIERE (c. 1520-1592), French soldier and diplomatist, was born in Touraine. After some service in the French army in Italy he entered the service of the Cardinal of Lorraine. He was employed on a series of important diplomatic missions, and after the death of Francis II. accompanied his widow, Mary Stuart, to Scotland in 1561. During that year he sought in vain to effect a recon ciliation between Mary and Elizabeth. During the next ten years he was employed alternately in the army and in diplomacy. In 1572 he was sent to England by Charles IX. to allay the excite ment created by the massacre of St. Bartholomew, and the same year he was sent to Germany and Switzerland. Two years later he was reappointed by Henry III. ambassador to Queen Elizabeth, and he remained at her court for ten years. Castelnau used his influence to promote the marriage of the queen with the duke of Alencon, but Elizabeth made so many promises only to break them that at last he refused to accept them or communicate them to his Government. On his return to France he found that his château of La Mauvissiere had been destroyed in the civil war; and as he refused to recognize the authority of the league, the duke of Guise deprived him of the governorship of Saint-Dizier. He was thus brought almost to a state of destitution. But on the accession of Henry IV., the king, who knew his worth, and was confident that although he was a Catholic he might rely on his fidelity, gave him a command in the army, and entrusted him with various confidential missions.
Castelnau died at Joinville in 1S92. His Memoires rank very high among the original authorities for the period they cover, the II years between 1559 and 1570. They were written during his last embassy in England for the benefit of his son ; and they possess the merits of clearness, veracity and impartiality. They were first printed in 1621; again, with additions by Le Laboureur, in 1659; and a third time, still further enlarged by Jean Godefroy, in 1731. Castelnau translated into French the Latin work of Ramus, On the Manners and Customs of the Ancient Gauls. Various letters of his are preserved in the Cottonian and Harleian collections in the British Museum.