Usellus is situated in the interior of Sardinia, about 25 km from the Gulf of Oristano on the west coast, and the same distance south of Forum Trajani (modern Fordongianus). Its name is not found in the Itineraries, and the only ancient author who mentions it is Ptolemy, who erroneously places it on the west coast of the island: but the existing ruins, together with the continuity of the name, leave no doubt of its true situation. It is about 5 km northeast of the modern town of Ales. Ptolemy styles it a colonia, and this is confirmed by an inscription on a bronze tablet of 158 CE (a tabula patronatus, setting forth that M. Aristius Balbinus had accepted the position of patron of the town for himself and his heirs) that speaks of the place as Colonia Julia Augusta Uselis. It would hence appear probable that the colony must have been founded under Augustus, though Pliny asserts that Turris Libisonis (modern Porto Torres) was the only colony in Sardinia at his time (79 CE, hence after Augustus' reign) It may be that civic rights were obtained from Augustus.