The least darter averages about one to two inches in length. It is green-brown on the upper body. There are eight to 12, small, rectangular blotches along the side. A teardrop mark under the eye, bars on the tail and second dorsal fin and specks of brown pigment over the entire body can also be seen. The lateral line is absent or consists of no more than two pores. There are two spines in the anal fin and no scales on the cheeks or belly. The breeding male is green-brown with dark spots on the head and body, red dots in the first dorsal fin and red-orange anal and pelvic fins. Tubercles (bumps) develop on the pelvic fins of the breeding male.
The least darter may be found in the northeastern one-fourth of Illinois. It lives in natural lakes and small, shallow streams that have abundant amounts of aquatic plants. Spawning occurs in May in shallow water over plants. The eggs are deposited one at a time and scattered on the plants, about 30 eggs per day. The least darter feeds on small crustaceans and immature insects it finds among the plants.