The widemouth gambusia (Gambusia eurystoma) is a species of fish in the family Poeciliidae of the order Cyprinodontiformes.

[2] It is endemic to Mexico, specifically to the Baños del Azufre (Grijalva River basin) near Teapa, Tabasco.

This prevents most animals from living in them; the only other fish found in the toxic sections of Baños del Azufre is the sulphur molly (Poecilia sulphuraria).

[3] This species reaches a maximum overall length around 3.5 cm (1.4 in).

[2] Little is known about G. eurystoma, but the IUCN classifies it as Critically Endangered on the basis of a very small (less than 250 individuals) and rapidly falling population and a small, localized, and diminishing geographical distribution.