Bryconaethiops yseuxi can be distinguished from all other known Bryconaethiops species based on the following unique combination of characters: a high number of scale rows between anterior origin of dorsal fin and lateral line (7.5 vs. 6.5 or less); a low number of gill rakers on the first gill arch; and a long dorsal-fin and anal-fin bases, respectively 15.7-17.6% and 22.7-25.9% of standard length (Ref. 74421). Body deep and somewhat laterally compressed (Ref. 74421). Mouth terminal; upper jaw with three rows of premaxillary teeth; external row composed of two small median teeth with a small lateral cusp at their outer side; a median row with 6 or 7 teeth; and an inner row of 8 pluricuspid teeth with their cusp on a median row and without an anterior cusp or row of cusps as generally found in the other Bryconaethiops species (Ref. 74421). Lower jaw with an external series of 6 large pluricuspid teeth and an inner row of two median caniniform teeth (Ref. 74421). Lateral line scales 34-39; longitudinal line scales 36-40 (Ref. 74421). Head and body of live mature male specimens silvery with a pinkish hue on sides of the body; upper side of head green-olive; humeral spot present but not well marked; adipose-fin whitish transparent, blackish at base (Ref. 74421). After preservation, body yellowish white; darker above lateral line with scales above lateral line faintly dark brown on their basal part; fins whitish, transparent in overall appearance; humeral spot not well distinguishable, or even regularly absent (Ref. 74421).