Ayolas, Juan de (hwän dā äyōˈläs) [key], d. 1537?, Spanish conquistador, explorer of the Río de la Plata country. He accompanied Pedro de Mendoza on his expedition of 1535–36. Sent to look for provisions, he sailed up the Paraná River and founded a fort called Corpus Christi. Later, leaving Domingo Martínez de Irala at a port called Candelaria, he went up the Paraguay River in search of a route to Peru. He fought the Guaraní, possibly at the site of Asunción, crossed the Chaco plain to the mountains, and is said to have been killed by the Native Americans on his return to Candelaria.
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