They are mentioned in Ptolemy's Geographia, as being west of the Silures. He mentions two of their towns, Moridunum (modern Carmarthen) and Luentinum (identified as the Dolaucothi Gold Mines near Pumsaint, Carmarthenshire). They are not mentioned in Tacitus' accounts of Roman warfare in Wales, which concentrate on their neighbours the Silures and Ordovices.
Vortiporius, "tyrant of the Demetae", is one of the kings condemned by Gildas in his 6th century polemic De Excidio et Conquestu Britanniae. This probably signifies the sub-Roman petty kingdom of Dyfed.