PIO I, POPE, saint. - In the Roman Martyrology he is commemorated on 11 July; it was Ado who first inserted him in his Martyrology. He succeeded Hyginus, but the chronology is uncertain, c. 140–155. At his death he is said to have been buried near the tomb of St Peter.
From the so-called Muratorian Fragment it is known that he was the brother of Hermas (q.v.), the author of the Shepherd. During his pontificate the Gnostics Valentinus and Cerdon spread their doctrines in Rome, though the latter for a time returned to Catholic communion.
Later and legendary hagiographical traditions, also taken up by the author of the Liber Pontificalis, make him a native of Aquileia and connect him with the title of Pudentiana (q.v.).
BIBL.: Eusebius, Hist. ecc., IV, 11; Lib. Pont., I, pp. 132 ff.; Acta SS. Julii, III, Paris 1868, pp. 170–172; Martyrologium Romanum, p. 281 ff.