Diagnosis: Males of Aphyosemion bitteri can be distinguished from all other congeners by presenting a maze-like red pattern on the caudal fin; they also differ from all the other species of the genus, except members of the A. grelli species group, by having the basal two thirds of the unpaired fins of females yellow, and the marginal third greyish (Ref. 116793). Males of A. bitteri can be easily distinguished from males of A. grelli by a denser red pigmentation on the flanks consisting of four parallel horizontal lines becoming wider posteriorly, vs. two or three parallel horizontal lines of isolated red dots; by the dense red pigmentation of the unpaired fins, consisting of four horizontal bands on the dorsal fin, undulating coalescent red lines parallel to the fin rays on the caudal fin, and three horizontal red lines on the anal fin, sometimes coalescent and in some specimens forming a pattern similar to that of the caudal fin, vs. no red pigmentation on the unpaired fins or a few isolated red dots on the central part of the caudal; by having more scales in the circumpeduncular series in both sexes, 13-14 vs. 12; less anal fin rays, 11-12 vs. 13-14; and interrupted neuromast series in the median longitudinal series, vs. neuromast series in the median longitudinal series complete (Ref. 116793). Males of A. bitteri can be easily distinguished from A. mengilai by the dense red pigmentation of the unpaired fins, vs. sparse red pigmentation; by the continuous parallel horizontal red lines on the caudal peduncle wider than the interspaces, vs. interrupted red lines consisting of short series of small dots, in some cases forming a reticulated pattern; by having less dorsal and anal fin rays in both sexes, 9-10 vs. 13-14 and 11-12 vs. 15-16, respectively; by less scales around the caudal peduncle, 13-14 vs. 15-18; by less scales in the transverse series, 9-10 vs. 11-12; by interrupted neuromast series in the median longitudinal series, vs. neuromast series in the median longitudinal series complete; and by a distinct maximum standard length, with A. bitteri attaining less than 30 mm standard length versus more than 40 mm standard length for A. mengilai (Ref. 116793). Males of A. bitteri can be easily distinguished from males of A. escherichi, known from the same geographical area, by a dense red pigmentation on the flanks, consisting of four parallel horizontal lines becoming wider posteriorly which are as wide as the interspaces and fade into the red pigmentation of the caudal fin, vs. three to four very regular parallel horizontal lines of coalescent red dots with the interspaces wider than the lines ending at the posterior edge of the caudal peduncle; by the narrow dark grey dorsal and ventral margins of the unpaired fins, vs. yellowish and wider with red submarginal bands; and by the red pigmentation of the unpaired fins, forming a maze-like pattern, vs. numerous isolated red dots on the median part of the caudal and the basal two thirds of the anal fin (Ref. 116793).

Description: A small sized, slightly laterally compressed species; dorsal profile slightly convex, slightly concave to nearly straight on the caudal peduncle; maximum body depth approximately at the level of the pelvic fins; ventral profile slightly convex, slightly concave to nearly straight on the caudal peduncle (Ref. 116793). Snout slightly rounded, mouth directed upwards, lower jaw longer than upper jaw; posterior end of the rictus at the same level as the centre of the eye (Ref. 116793). Caudal fin truncate to subtruncate; dorsal and anal fins located posterior to the midbody, tips rounded; small dorsal fin with 9-10 fin rays, first dorsal-fin ray inserts above the 4th to 5th anal-fin ray; anal fin with 11-12 rays; caudal fin with 27-28 rays; pectoral fins with 16 rays; pelvic fins with 6 rays (Ref. 116793). Anterior neuromast series of the 'open' type; anterior supraorbital series with one neuromast; central supraorbital series with two neuromasts; posterior cephalic neuromast series with two to three neuromasts; preopercular canal with six pores; one neuromast on each scale of the median longitudinal series, some neuromasts missing on the caudal peduncle (Ref. 116793). Scales cycloid, body entirely scaled except the ventral surface of the head; frontal squamation of the G-type; 28-30 scales in the mid-longitudinal series, with three to four scales posterior to the hypural plate; 9-10 transversal scales, 13-14 scales around the caudal peduncle (Ref. 116793). Premaxilla and dentary with many irregularly distributed unicuspid, slightly curved teeth of different size; five brabchiostegal rays; vomerine teethnot present, anterior border of the vomer thickened; lateral process of the post-temporal not present; single anterodorsal process of the urohyal (Ref. 116793).

Colouration: Males in life: flanks metallic blue-green; body darker dorsally the ventrally; three pale red streaks on the opercle, obliquely inclined upwards at an approximate 45° angle; lower jaw opaque red; sides of body with three horizontal stripes staring behind the opercle and ending on the posterior margin of the caudal peduncle, anteriorly consisting of small red coalescent dots, gradually becoming wider towards the caudal peduncle, so that the metallic blue-green interspaces are narrower than the stripes; a fourth ventral stripe starting below the insertion of the pectoral and ending at the posterior end of the basis of the anal fin, more irregular and interrupted; pectoral fin hyaline with an iridescent blue-green margin; pelvic, dorsal, and anal fin blue-green; dorsal fin with three regular horizontal red bands slightly wider than the interspaces, followed by a fourth distal irregular dark grey band; in some specimens, red pigmentation forming instead 4 to 5 more or less continuous rows of red spots; caudal fin with dense red pigmentation in the form of undulating coalescent red lines parallel to the fin rays on iridescent, blue-green background, suggesting a maze-like red pattern; anal fin with colour pattern similar to the dorsal fin, but with less numerous regular red lines that can be interrupted and coalescent; pelvic fin blue-green with some red dots (Ref. 116793). Females in life: flanks greyish-brown with four regular horizontal lines formed by reddish dots; all lines start behind the opercle, the two dorsalmost lines reaching the edge of the caudal peduncle, the line below these ends on the vertical through the insertion of the dorsal fin, the ventralmost line consisting of only four to five reddish dots; ventral area whitish to yellowish; the scales of the median part of the flanks show narrow dark edges forming a subtle reticulated pattern; lower jaw pale red; pectoral fins hyaline; pelvic, dorsal, anal, and caudal fins yellow with a broad, greyish margin along their distal third; dorsal fin with a basal row of isolated red dots, short red streaks parallel to the fin rays at their central portion; upper procurrent caudal fin rays with two to three small red dots (Ref. 116793).