Diagnosis: Absence of a dark submarginal band in the dorsal fin distinguishes Petrotilapia flaviventris from males of P. microgalana, P. genalutea, P. nigra, P. chrysos, P. mumboensis, and P. pyroscelos, which have such a band (Ref. 87181). Adult males of P. flaviventris are yellow on ventral and mid flank with scales outlined in blue; the dorsal one-third is blue to gray with yellow and orange highlights; cheeks and throat are yellow to orange; males of P. tridentiger are light blue with dark blue bars; while those of P. xanthos are yellow; adult male, and female, P. palingnathos have distinctly retrognathous jaws and can thus be distinguished from those of P. flaviventris (Ref. 87181). Females of P. flaviventris are yellow brown with interrupted black horizontal stripes and with a background colouration fading to dark brown dorsally, which distinguishes them from those of P. tridentiger, which are brown, and from P. chrysos and P. microgalana, which are golden yellow; females of P. flaviventris are further distinguished from those of P. xanthos, which are light brown (Ref. 87181).

Description: Body depth at origin of dorsal fin between 36.3-39.0% of standard length; dorsal head profile concave between snout tip and interorbital (Ref. 87181). Horizontal and vertical eye diameters 28.0-34.7% of head length and 28.6-33.7% of head length, respectively; preorbital depth 24.2-28.6% of head length; snout 32.6-39.9% of head length; isognathous jaws and thickened lips; teeth on lower jaw in 10-18 rows with outer row and inner rows tricuspid (Ref. 87181). Dorsal fin with XVII-XIX spines and 8-9 rays; anal fin with III spines and 7-8 rays; first dorsal-fin spine less than half the length of eighth spine; caudal fin subtruncate to emarginate; depressed pelvic fin not reaching anal fin in females, reaching first anal spine in males; pectoral fin round and paddle-shaped (Ref. 87181). Flank scales large, ctenoid; cheek with 3-4 rows of scales; lateral line with 30-31 pored scales (Ref. 87181).

Colouration: Live males with yellow cheek, ventral preopercle, opercle, and throat; preopercle and opercle with blue highlights; interorbital dark blue; flank blue and yellow; ventral one-third yellow, mid one-third yellow with scales outlined in blue and yellow highlights; breast and belly yellow; dorsal fin yellow and blue with blue membranes; caudal fin with blue rays and yellow membranes; anal-fin spines and membranes blue/black, anterior rays blue/black, posterior rays blue with 1-4 ocelli, anal-fin membrane blue and black; pelvic fin yellow with blue leading edge; pectoral fin with yellow rays and clear membranes (Ref. 87181). Live females with dark brown head, black opercle with blue green highlights and gray throat; flank yellow brown with interrupted black stripes with 8-9 faint vertical bars, and fading to dark brown dorsally; breast and belly light gray; dorsal-fin spines brown, proximal two-thirds of membranes brown, distal one-third light blue; gray brown lappets, and membranes of rays with brown spots; caudal fin with brown rays and blue gray membranes; proximal two-thirds light brown, distal one-third clear without ocelli; membranes of first two rays of pelvic fin yellow brown, remaining membranes clear; pectoral fin with brown rays and clear membranes (Ref. 87181). Preserved males with brown head, light brown cheek, black opercle, and light brown to yellow throat; flank of ventral and mid-side light brown to yellowish and scales outlined in brown with 6 faint brown vertical bars; flank dorsally dark brown with dark spots; belly and breast light brown to white; dorsal fin light brown to white with clear spines, rays, and membranes; dorsal fin without black submarginal band; caudal-fin rays light brown with clear membranes; anal fin light brown with dark rays and spines with dark leading edge; pelvic fin light brown with clear spine, rays, and membranes; pectoral fin with light brown rays and clear membranes (Ref. 87181). Preserved females with dark brown head, black opercle, brown cheek, and brown to light brown throat; flank brown with mid-lateral and dorso-lateral row of dark blotches and 8-9 black bars; belly yellow to whitish, breast brown to yellow; dorsal fin light brown with clear membranes and without submarginal band; brown spots on rays; caudal fin with gray rays, clear membranes, and black blotches; anal fin with brown rays and spines with clear membranes; pectoral fin with light brown rays, clear membranes; pelvic fin with brown spine and anterior rays; posterior rays and membranes clear (Ref. 87181).