Nannostomus unifasciatus, (from the Greek: nanos = small, and the Latin stomus = relating to the mouth; from the Latin: unifasciatus = one band),[1][2] commonly known as the one-lined pencilfish, is a freshwater species of fish belonging to the genus Nannostomus in the characin family Lebiasinidae.

They are popular in the aquarium trade due to their small size, beautiful color pattern, unique swimming posture and relative hardiness.

[8] N. unifasciatus, is broadly distributed throughout the Amazon basin, in Brazil, the Guiana Shield, Colombia, Venezuela, and northern Bolivia.

[7] Consensus was finally achieved when, in his seminal paper on the genus Nannostomus in 1975, Dr. Stanley Howard Weitzman restored Steindachner's taxonomy and expanded upon it, reaffirming the status of N. unifasciatus and N. eques as distinct and individuated species, and placing N. unifasciatus and all of its congeners in one genus, Nannostomus.

[6] Nannostomus unifasciatus commonly inhabits slow-moving tributaries, small rivers, and swampy areas throughout its substantial range.

[6] Though the one-lined pencilfish is a small fish, with adults not reaching more than 7 cm in length, it is nevertheless one of the largest species of Nannostomus.

[8] Most all forms have red or orange pigment of varying intensity in the lower quadrant of the caudal, ventral to the black stripe.

[1] It is an appropriate community aquarium species, if tankmates are of similar size and demeanor, and will thrive if kept in soft, mildly acidic water, low nitrate levels, and at temperatures between 72F and 82F.

Unlike its congener, N. eques, which prefers the upper strata of the aquarium, N. unifasciatus tends to congregate in the mid to lower reaches.