Fique fibers are extracted from Furcraea spp. leaves, with 5% average mass yield, using mechanical decortication. Juice, pulp and tow, the by-products of this process, amount 95% of the leaf weight and are considered waste. We extracted cellulose nanofibrils (CNF) from Fique tow, via ultrasound-assisted TEMPO followed by mechanical disintegration with sonication. Fique CNF exhibit diameters around 100?nm, degree of oxidation (DO) of 0.27 and surface charge density (?) of 1.6?mmol/g. Fique CNF aqueous suspensions show optical birefringence and high colloidal stability due to a high ? potential (?53?mV). The morphology, chemical structure, crystallinity and phase transitions of Fique CNF were studied using FESEM, IR-ATR, XRD and TGA. We observed that the delignification pretreatment and the TEMPO reaction assisted by ultrasound significantly increase Fique CNF ? and ? potential, in contrast with the oxidation carried out without ultrasound or with raw (lignified) tow.