The fantail darter averages about one and one-half to three inches in length. It is green-brown or brown. There is a large, black scale on the side just above the pectoral fin base. The caudal (tail) fin is marked with black bands. Dark spots along the sides give the appearance of horizontal stripes. A dark stripe extends in front of and behind the eye. The lower jaw projects beyond the upper jaw. There are no scales on the head. The first dorsal fin is much shorter than the second dorsal fin. There are two spines in the anal fin. The lateral line is short, ending toward the front of the second dorsal fin. The breeding male is dark green-brown. He has black bars in the caudal fin, dark speckles in the second dorsal fin, gold knobs on the dorsal spine tips and a dark edge on the first dorsal fin.
The fantail darter may be found in the northern two-thirds of Illinois plus Jackson and Union counties. It lives at the bottom of fast, rocky riffles in creeks and small rivers, usually in shallow water. Spawning occurs in April and May. The knobs on the dorsal fin of the breeding male are used to clean the nest site and to clean the eggs after they have been deposited. Both the male and female turn upside down during egg deposition, with the eggs placed under a rock. About 45 eggs are laid per spawning with spawning occurring as many as five times per season. The male stays with the eggs after they have been laid. Hatching is determined by water temperature, usually within 14 to 35 days. The fantail darter eats immature insects and other aquatic arthropods.