Diagnosis: Marcusenius lambouri belongs to the group of species of Marcusenius with eight large circumpeduncular scales (Ref. 124516). Within this group, it can be distinguished from M. dundoensis, M. fuscus and M. sanagaensis by its low number of lateral line scales, 44-46 vs. 51-61, 54-61 and 49-56, respectively; and from M. intermedius, M. moorii and M. verheyenorum by its high number of lateral line scales, 44-46 vs. 38-43, 40-43 and 38-43, respectively (Ref. 124516). It differs from M. ghesquierei and M. kutuensis by its low number of dorsal-fin rays, 19-23 vs. 32-34 and 27-31, respectively; and from M. schilthuisiae and M. wamuinii by its lower number of anal-fin rays, 23-28 vs. 32-34 and 30-32, respectively (Ref. 124516). Finally, it differs from M. kaninginii by a high total number of vertebrae, 44-45 vs. 43-44, shallow caudal-peduncle depth, 29.6-38.2% of its length vs. 38.4-45.1%, the absence of a longitudinal black band along the flanks vs. band present, and a medial black line on each of the flank scales vs. absence of such a line on the flank scales (Ref. 124516).

Description: Elongated body, with upper profile of head slightly curved; middle body depth higher than its depth at base of dorsal and anal fins (Ref. 124516). Mouth terminal, upper level of upper jaw located at lower level of eye; eye located on anterior part of head; bulky and rounded submental swelling; lower nostril more posteriorly on head than upper one (Ref. 124516). Three to five bicuspid teeth on upper jaw, six on lower jaw (Ref. 124516). Short pectoral fin, inserted just posteriorly to base of pelvic fin; predorsal distance more or less equal to pre-anal distance; dorsal fin slightly shorter than anal fin, starting from level of third to fifth anal-fin ray and ending before level of caudal fin; dorsal-fin rays 19-23, anal-fin rays 23-28, pectoral-fin rays 10 (Ref. 124516). Scales on lateral line 44-46; scales between dorsal and anal fins 13-17, scales between dorsal fin and lateral line 10, scales between pelvic fin and lateral line 8-11 (Ref. 124516).

Colouration: In life, body, head and upper part of caudal peduncle brownish; posterior flank with a black vertical stripe from between anterior part of dorsal-fin base, from about first ray onwards, towards anterior part of slightly more anterior anal-fin base, from about second to ninth ray onwards, and with a width covering two to three horizontal scale rows; in addition, a dark brownish or black horizontal midlateral stripe, of about 20% of a scale in width, is also present on each of the flank scales between the first one above the anterior base of the anal fin and the 12th one below the anterior base of the dorsal fin and forming long horizontal lines over the flanks from behind the head up to the anterior base of the caudal peduncle; pectoral fin brownish; dorsal, anal and caudal fins yellowish brown (Ref. 124516). In preserved specimens, head, dorsal part of body and caudal peduncle brownish yellow to dark brown; pectoral, pelvic, dorsal and caudal fins brownish yellow; dark brownish or black vertical stripe and horizontal midlateral stripes still visible after preservation but fading with time (Ref. 124516).