A moderately slender darter with 6 or 7 dark crossbars on the back. The upper lip is often separated from the snout at the midline by a continuous shallow groove. The forward half of the belly lacks scales; the back half is covered by scales of normal size and shape. Gill covers broadly connected by a membrane across throat. Lateral line complete.
Back and upper sides mottled green. Midside with a series of dark green blotches. Lower parts yellowish-white with scattered dusky spots or blotches. All fins marked by narrow brown lines and spots. Males more brightly colored and more boldly marked than females.
Similar to banded darter, but the harlequin has the second and fourth dark crossbars on back narrower than the adjacent bars; body at base of tail fin with large blotches; cheek and gill cover usually without scales; and pectoral fins very long, extending backward past tips of pelvic fins.