Abstract : This paper proves three Organon manuscripts copied by Theodore Agallianos circa 1442 : Vat. gr. 1777, Par. gr. 1919, ff. 124-163 ; and Rep. I 68, f. 1-30. The author offers multifarious evidence that those three manuscripts, which in reality are three broken codicological units, belonged originally to one and the same unit (= MS β). The origins of MS β and the main stages of its life are analysed. The author argues that MS β depends on Neap. III D 37, a fourteenth-century codex written by Isaac Argyros, and that it passed through the hands of the Cretan scribe George Tribizias, who restored three missing quires circa 1465.