Flagfish are small, robust fish, 6 centimetres (2.4 in) in length, with a truncated snout which has been compared to that of a bulldog. They have rounded fins with the dorsal and anal fins positioned posteriorly and adjoining the caudal fin. Females have an obvious spot on their flanks and a clear spot near the posterior end of the dorsal fin, this has an opaque white margin. The fins may be show a slight reddish colour but this comes and goes in any individual but the reason for this is unknown. The females body is mainly olive but marked with turquoise scales. The common name of the flagfish is derived from the male due to the resemblance of its patterning to the flag of the United States. There is a dark rectangle on the shoulder which is imagined as the blue and white stars in the corner of the US flag and there are stripes along the flanks, some of which are red and others paler, albeit greenish rather than white. The male also has a dark spot positioned at the lower posterior corner of the dark rectangle. The olive-grey colouration and pattern of nondominant males, females, and juveniles underlies the more colourful pattern of the dominant males.