Diagnosis: Nothobranchius flagrans belongs to the N. brieni species-group, presenting the diagnostic characters in male colouration for this complex of the caudal and anal fins proximal portion spotted, distal part with light margin or light submarginal band and dark margin; absence of a dark distal margin on the dorsal fin; absence of spots on head; and absence of black posterior margin of scales (Ref. 98826). Nothobranchius flagrans is distinguished from all other species of this complex, with the exception of N. hassoni, by having the anal and caudal fins with broad orange-red submarginal band (Ref. 98826). It is, however, further distinguished from N. hassoni by having a dorsal fin with a light blue distal margin vs. absent; and anal and caudal fins with dark grey distinct margin vs. red-brown (Ref. 98826). Furthermore, within the N. brieni species-group, N. flagrans is characterised by the following unique combination of morphometric characters: prepelvic length 49.5-52.6% of standard length; head depth 86-92% of head length; and caudal peduncle length 151-161% of its depth (Ref. 98826).

Description: General body shape robust, laterally compressed and deep; greatest body depth at vertical in front of pelvic-fin origin; greatest body width at pectoral-fin base (Ref. 98826). Dorsal profile convex from tip of snout to base of last dorsal-fin ray, straight to slightly concave on caudal peduncle; ventral profile convex from lower jaw to base of last anal-fin ray, straight to slightly concave on caudal peduncle (Ref. 98826). Head short, laterally compressed, deeper than wide; snout slightly pointed, smaller than eye diameter; mouth supraterminal, slightly oblique in profile (Ref. 98826). Jaws subequal, lower jaw longer than upper, posterior end of rictus at same level or slightly ventral to centre of eye; premaxilla and dentary with many irregularly distributed unicuspid, slightly curved teeth, larger teeth of about 70 µm at outer row of lower and upper jaws (Ref. 98826). Orbit large, in anterior half of head; branchiostegal membrane projecting posteriorly from operculum (Ref. 98826). All fins soft-rayed: dorsal-fin origin anterior to anal-fin origin, both fins originating posterior to mid-length of body; extremity of dorsal and anal fins rounded, with small contact organs in form of papillae on fin rays and distal margin with short filamentous rays; posterior extremity of dorsal fin reaching caudal-fin base; dorsal fin with 16-18 rays; anal fin with 16-19 rays; pectoral fin subtriangular, insertion at about vertical or slightly posterior to margin of opercular opening, base slightly oblique, upper fin rays placed slightly anteriorly to lower fin rays, tip reaching or slightly overlapping base of pelvic fin; pelvic fin subabdominal, origin at about mid-length of body, short, bases medially separated, tip reaching urogenital papilla; caudal fin subtruncate (Ref. 98826). Scales cycloid, body and head entirely scaled, except for ventral surface of head; scales in mid-longitudinal series 27-29 plus 2 or 3 small scales on caudal-fin base; transverse rows of scales in front of dorsal-fin origin 10-11; scale rows around caudal peduncle 10-12 (Ref. 98826). Cephalic squamation pattern variable, some specimens presenting irregular G-type, and with E-scales and F-scales overlapping each other at the median lateral margin; nostril in front of eye, with single oblique aperture; anterior neuromasts separate in two grooves; cephalic sensory system at preorbital level in two discontinuous shallow grooves, with two and four exposed neuromasts, whereas at supraorbital level in two curved grooves, with four and two exposed neuromasts; one neuromast on each scale along trunk mid-longitudinal series (Ref. 98826).

Colouration: Live males: scales on trunk and head light blue with red-brown posterior margin, forming reticulated pattern on body; scales on abdomen faint light blue; snout and dorsal portion of head red-brown, throat red-brown with light blue spots; posterior scale margins on post-orbital portion of opercle creating red-brown vertical bars; exposed part of branchiostegal membrane red-brown; iris light yellowish, with dark grey vertical bar through centre of eye; dorsal fin red-brown with irregular light blue spots proximally, becoming a light blue striped pattern in the median part, parallel to fin rays, and with red-brown submarginal band and distinct narrow light blue distal margin; anal fin with alternating narrow light blue and red-brown proximal bands and narrow yellow to cream median band, followed by a broad orange-red submarginal band and a distinct dark grey distal margin; caudal fin red-brown with light blue spots followed by a narrow light blue subproximal band, and with broad orange-red submarginal band and distinct dark grey distal margin; pelvic fin red-brown, with light blue spots and light blue distal margin; pectoral fin hyaline with light blue posterior distal margin (Ref. 98826). Live females: scales on trunk and head pale grey-brown, darker on dorsum and lighter to silver on venter; faint dark grey reticulation on dorsal and posteroventral portions of flank; blue iridescence on trunk and opercular region; iris golden, with faint dark grey vertical bar through centre of eye; all fins hyaline (Ref. 98826).