Pictures are of adults. 

Xiphophorus pygmaeus, the pygmy swordtail, is a poeciliid fish from northeastern Mexico. It is the smallest of the swordtails. The male's sword is barely visible and the species is often called the swordless swordtail. It is sometimes kept in home aquaria, but is a rather delicate species.

Females grow to 4.5 cm (1.8 in), while males attain 3.5 cm (1.4 in). The male's sword is only 1-2 mm long. The species was originally thought to consist only of small-sized males (all less than 29 mm long and most less than 24 mm), but in 1994 it was discovered that two populations contain large males.

The base color of the fish is gray-brown. Both small and large males can be yellow, but this color morph is very rare in nature because such flashy individuals are easy prey. No yellow females exist, and the trait appears to be sex-linked: male descendants of yellow males tend to be yellow too. Males are generally more intensely colored than females.