Among the medieval hagiographical writings derived from the British Isles none enjoyed greater popularity throughout the Middle Ages than the Navigatio Sancti Brendani Abbatis (= NB). This celebrated prose work, a typical product of the Othonian period, has come down to us in more than a hundred MSS in various versions. It embodies the adventurous sea-story of the Irish Abbot St. Brendan, one of the great sixth-century founders of monasteries. In structure, the NB consists actually of three parts: a brief introduction comments on St. Brendan's descent, youth, ascetic life, and early monastic foundations; the main body reports some twenty-six adventures which he and his fourteen companions encountered in their search for the terra repromissionis or paradisum terrestre, the tír tairgirne of the ancient Celts; finally, a terse epilogue narrates his life after his return and subsequent happy death. While the main body of the NB, the sea-voyage proper, is uncompounded and has been modeled after Old Irish sea-tales, known in Celtic literature as immrama, both the introduction and epilogue, necessary to give the story the appropriate frame, represent incidents culled from the Vita Sancti Brendani (= VB), which has come down to us in various Irish and Latin recensions. These two narratives have over the centuries been combined by several medieval compilers into a single story in a more or less artistic way. Consequently, the student of the NB is ultimately confronted with that much feared and confusing type of Brendaniana, called conflated texts, which in view of the absence of clearly drawn lines between the contents of the VB and NB, have for centuries offered vexing problems to researchers. One of the minor, but nevertheless irritating, results of these fusions is the misleading caption ‘Vita’ Sancti Brendani, exhibited by a goodly number of NB-MSS, which has misled many cataloguers, medieval and modern, to list the Navigatio as a Vita. Thus, not less than half of all NB-MSS sail in the maelstrom of medieval literature under a false flag. A most peculiar Latin NB-MS, showing the same misleading caption Vita Sancti BrendaniAbbatis, is codex 256 of the Biblioteca Nacional de Lisboa, Lisbon, Portugal. This MS, hitherto unavailable to research, is of signal importance for the history of the Vita, the Navigatio, and above all, for the Old French translations of the Navigatio with their re-translations into Latin, so unique in medieval literature.