Diagnosis: Nothobranchius rungwaensis is distinguished from all other species of the genus by the following characters in males: body colouration light blue to blue-green with red-brown scale margins; anal fin yellow with a narrow red-brown submedial band, a broad yellow medial band with red to red-brown spots, and a broad black distal band; caudal fin light grey with slender semi-translucent dark grey distal band; pelvic fins light blue-grey, with broad yellow medial band with red-brown spots, and narrow black distal band (Ref. 122068). Furthermore, it differs from the other members of the Nothobranchius taeniopygus species group in central and western Tanzania by a head width 54% of head length vs. 56-73%, and 62% of its depth vs. 63-77%; interorbital width 38% of head length vs. 39-52%; postorbital length 46% of head length vs. 47-63%; snout length 22% of head length vs. 16-22%; and caudal peduncle length 119% of its depth vs. 121-169% (Ref. 122068).

Description: General body shape robust, laterally compressed and deep; greatest body depth at vertical in front of pelvic-fin origin, 33.% of standard length; greatest body width at pectoral-fin base with body progressively narrowing towards caudal-fin base (Ref. 122068). 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; caudal peduncle relatively deep, length 1.2 times of its depth; anus situated directly in front of anal-fin origin (Ref. 122068). Head modestly long, 32.5% of standard length, laterally compressed, deeper than wide, head width 62% of its depth; snout slightly pointed, smaller than eye diameter; mouth supraterminal, slightly oblique in profile; 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 conical, slightly curved teeth at outer row of lower and upper jaws; orbit large, 32% of head length, mostly in anterior half of head, snout to eye end length 54% of head length, in dorsal portion of head side; branchiostegal membrane projecting posteriorly from opercle (Ref. 122068). 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 15 rays; anal fin 14 rays; pectoral fin subtriangular, insertion 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, with 17 branched rays, plus 2 to 3 unbranched smaller rays at dorsal and ventral origins (Ref. 122068). Scales cycloid, body and head entirely scaled, except for ventral surface of head; no scales on dorsal and anal fins; scales in mid-longitudinal series 31 plus two or three small scales on caudal-fin base; transverse rows of scales in front of dorsal-fin origin 11; scale rows around caudal peduncle 14 (Ref. 122068). Cephalic squamation pattern G-type; frontal neuromasts separate in two rows of shallow grooves behind the upper lip, with one neuromast in each groove; cephalic sensory system at supraorbital level fragmented into two discontinuous portions, in shallow grooves with two exposed neuromasts both in anterior and posterior portions; supratemporal level with five exposed neuromasts in undivided, curved shallow groove; preorbital canal in shallow groove with two exposed neuromasts; infraorbital level with series of about ten small buttons at ventral and posterior margin of orbit; postorbital canal in deep groove with one exposed neuromast; preopercular system with exposed neuromasts in deep groove; mandibular canal in shallow groove with about ten small neuromasts; one neuromast on each scale along trunk mid-longitudinal series (Ref. 122068).

Colouration: Live male: scales on trunk and head light iridescent blue to blue-green with red-brown margins, forming a regular reticulation pattern; scales on abdomen faint blue to white with less pronounced margins; snout, frontal and dorsal portions of head blue to blue-grey; throat pale blue; exposed part of branchiostegal membrane cream to white; iris golden; background colour of dorsal fin grades from light blue-grey to yellow-grey basally through light brown-pink in the proximal zone, to light green and golden distally, with irregular brown dots and stripes, often elongated and aligned parallel to the rays, and becoming finer distally; black markings may be present on membrane between first 3-4 anterior fin rays; anal fin with a narrow light brown to yellow basal and proximal zone overlaid by sparse red-brown spots, followed by a narrow deep red-brown submedial band; a medial zone that comprises a broad golden yellow zone overlaid with deep red to red-brown spots that tend to be concentrated in the outer part of the zone and may be somewhat elongated parallel to the rays; the distal zone is broad and black; a very narrow, poorly developed, irregular pale yellow subdistal band may be present; caudal fin grey with faint subdistal blue-green iridescence and slender, transitional dark grey distal band; pelvic fins mimic pattern of anal fin with narrow light blue-grey to brown basal zone, a broader yellow medial zone with red markings, and a narrow black distal band; pectoral fins dominantly hyaline with light blue margins (Ref. 122068). Live female: trunk and head light grey-brown, darker on dorsum and lighter ventrally; scales with a very narrow grey margin; scale centres iridescent silver to very pale blue; all fins hyaline; iris golden (Ref. 122068).