Ustou is a commune in the Ariège department in southwestern France.The commune of Ustou comprises several villages: Saint-Lizier, Le Trein, Sérac; and numerous hamlets, including: Escots, Bieille, Rouzes.PopulationInhabitants of Ustou are called Ustouens.Sights The Château de Mirabat, a Middle Ages castle, known to be in ruins in the 14th century, is in the communes of Seix, Oust and Ustou.