The harlequin darter has two spots at the base of the caudal fin. Its belly and the underside of the head have dark speckles. The back is yellow to green with six to seven dark crossbars, and there are seven to 11 dark spots on each side. The first dorsal fin is dark red along the edge. A dark teardrop mark is present below the eye. The front half of belly is scaleless. The anal fin has two spines. The pectoral fins are very long. All fins have narrow dark lines and spots. This species may grow to three inches in length.
The harlequin darter can be found in Illinois only in the Wabash River in southeastern Illinois. It formerly lived in the Embarras River, too, but has not been found there since 1984. It prefers shallow water with a sand or gravel substrate where there are plenty of dead leaves and sticks. It eats small crustaceans and insects.