Michel Aco (both: mēshĕl´ äkō´), fl. 1680–1702, French explorer. He became La Salle's lieutenant, being favored by that explorer because of his courage, prudence, and wide acquaintance with Native American languages. When La Salle reached the mouth of the Illinois River on his famous voyage down the Mississippi, he sent Aco with two companions to explore the upper reaches of the Mississippi. One of the companions was Father Louis Hennepin, who in his Nouvelle Decouverte made himself the hero of the expedition. Near the Falls of St. Anthony, which they were the first Europeans to see, the three were captured by members of the Sioux tribe and were released only through the energy and influence of Daniel Greysolon Duluth. Little is known of Aco's subsequent life except that he was a trader on the Illinois for many years and that in 1693 he married the daughter of a Kaskaskia chief. His name also appears as Ako.