Nematolebias is a genus erected by Costa in 2002 with Nematolebias whitei as its type species. At the time Nematolebias did become a genus the type species was named Simpsonichthys whitei but was origanally described as Cynolebias whitei, Myers 1942.

The new species N. papilliferus, Costa 2002 and the, than named, Simpsonichthys myersi, Carvalho 1971, where added to this genus by Costa.

The acceptation of this genus is not supported by several scientists and this site will, for the time being, present the 3 species in the genus Simpsonichthys.

N. papilliferus is found in small freshwater temporary pools, 10-40 m of diameter, about 50 cm deep, in floodplains of streams and lagoons or along the vast floodplains of lower rio São João, which are sometimes close to the sea (about 100 m). Substrate of compact, soft reddish brown mud; water hyaline, yellow or dark orange brown, usually not turbid. These pools are dry twice a year, once during winter, usually from July to August, and again during summer, usually from February to March. During dry season, all fish die and eggs undergo in diapause within the substrate.

Nematolebias whitei occurs in small freshwater temporary pools in floodplains of streams and lagoons. These pools are shallow with compact, soft reddish brown muddy bottom and dry twice a year, during winter, usually from July to August, and during summer, usually from February to March. During dry season, all fish die and eggs undergo in diapause within the substrate. After first rains fall eggs hatch and young fry grows very rapidly to adulthood.

Nematolebias is dicussed and some scientist see no reason to exclude these 3 species from the genus Simpsonichthys.