Among the books of the prophets, the Book of the prophet Isaiah is the most quoted by S. Augustine. The article deals with Is 40: 6-8, as it is quoted within S. Augustine's Enarrationes in Psalmos, presenting the textual differences with the other works of S. Augustine, and it makes the comparison between these quotations and other Biblical Versions (LXX, Vulgate, Vetus Latina). It deals also with the main points of the Spiritual Theology that S. Augustine explains within his exposition of Is 40: 6-8.