Diagnosis: Hyphessobrycon natagaima differs from all other species of Hyphessobrycon with a dark lateral stripe that inhabit the Magdalena River Basin, H. poecilioides, H. proteus and H. ocasoensis, by the number of pored lateral-line scales (8 to 12 vs. 14-26); the number of scales between lateral line and pelvic-fin insertion (4 vs. 5 or 6), the number of teeth on the maxilla (1 vs. 0 in H. poecilioides, and 2 to 5 in H. proteus; except H. ocasoensis, which also has 1); a dark, interrupted lateral stripe that is not in contact with the caudal peduncle spot (vs. absence of caudal spot in H. poecilioides, lateral stripe continued that is in contact with the caudal peduncle spot in H. ocasoensis); a rhomboid shaped caudal-peduncle spot that continues on to middle caudal-fin rays (vs. absence of caudal peduncle spot in H. poecilioides and caudal peduncle spot round and not continued on to middle caudal-fin rays in H. ocasoensis); and presence of hooks on all fins in mature males (vs. males with hooks on anal, pelvic and pectoral fins); Hyphessobrycon natagaima differs from H. ocasoensis in addition to the characters mentions above by having 4 scale rows between lateral line and anal-fin origin (vs. 6); 3 or 4, mode 4 scale rows between the lateral line and the pelvic-fin insertions (vs. 6); 10 or 11, mode 11 predorsal scales (vs. 9); the number of dorsal-fin rays (i,9,i vs. ii,8,i); 18-20, mode 20 branched anal-fin rays (vs. 21-22, mode 22), and 11 branched pectoral-fin rays (vs. 12)(Ref. 118120).

Description: body deep and wide, greatest body depth between vertical through pelvic-fin insertions and dorsal-fin origin; dorsal profile of head straight from tip of upper lip to vertical through middle of orbit of eye, then convex to dorsal-fin origin; dorsal-fin base nearly straight, then convex to adipose fin and slightly concave to base of upper caudal-fin lobe; ventral profile of head convex from lower lip to anal-fin insertion then slightly concave to base of lower caudal-fin lobe; head and snout long, jaws equal, mouth terminal, lips soft and flexible, outer premaxillary tooth row not exposed; premaxilla with long lateral process, rounded over ethmoids, and 2 rows of teeth: outer row with 2* (24), 3 (20) or 4 (2) all tricuspid; inner row with 5 (46) pentacuspid teeth (with the last tooth tricuspid), that gradually diminish in size away from symphysis; maxilla long and narrow, its posterior margin straight but anterior margin convex, posterior tip reaches ventral border of 2nd infraorbital, with 1 (46) pentacuspid tooth; dentary with convex ventral margin, 4 (46) heptacuspid front teeth followed by 3* (32) or 4 (14) smaller tricuspid teeth; scales cycloid; lateral line with 8 (1), 9 (2), 10 (3), 11*(7) or 12 (33) pored scales; lateral scales including those with pores 31 (17), 32* (18) or 33 (11); 6* (43) or 7 (3) horizontal scale rows between dorsal-fin origin and lateral line, not including scale of predorsal series just anterior to 1st dorsal-fin ray; 4 scale rows (46) between anal-fin origin and lateral line; 4 horizontal scale rows (46) between pelvic-fin insertions and lateral line; predorsal scales 10*(44) or 11 (2); 4 scales in a single row on base of anterior anal-fin rays; dorsal-fin rays i,9,i (46); anal-fin rays iv,18 (3), 19 (20) or 20* (23); pelvic-fin rays i,6,i (46); pectoral-fin rays i,11,i (46); caudal-fin forked, upper and lower lobes pointed, similar in size; principal caudal-fin rays 10+9 (3); procurrent caudal-fin 10 (3); total vertebrae 32-33 (5); first gill arch with 20 rakers, 3 on hypobranchial, 10 on ceratobranchial and 7 on epibranchial; proximal pterygiophores of dorsal-fin rays inserted between neural spines 9 to 16; anal-fin with 21 proximal pterygiophores, the 1st 2 inserted between hemal spines 11 and 12, reaching ventral border of centrum of hemal spine 12; 5 elongate supraneurals with cartilage on upper and lower tips, inserted over 4th to 9th neural spines

Color in alcohol: body light brown, dorsum dark brown; conspicuous, rhomboidal caudal peduncle spot extends on to middle caudal-fin rays; flanks with dark stripe, posteriorly to humeral spot, interrupted and not in contact with caudal-peduncle spot, deeper at vertical through dorsal-fin origin; dark humeral spot, vertically elongate, covering 2 scales below pored lateral-line; posterior margin of scales with dark cromatophores; edges of dorsal and caudal fins dark; pectoral, pelvic and anal fins hyaline; anal fin with dark cromatophores on membranes; top of head dark brown (Ref. 118120).