The widemouth gambusia (Gambusia eurystoma) is a species of freshwater fish. It is a member of the family Poeciliidae of the order Cyprinodontiformes. It is native to Mexico.
This species reaches a maximum overall length of approximately 3.5 cm (1.4 in).
Little is known about G. eurystoma, but 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.