[5] Both small and large males can be yellow,[4] but this color morph is very rare in nature because such flashy individuals are easy prey.

[6] Xiphophorus pygmaeus was discovered in Rio Axtla, a tributary of Río Pánuco, by Myron Gordon in 1939.

At the type locality, the fish were found in swift water among a dense stand of a Vallisneria-like plant along a steep bank.

X. montezumae and X. variatus are found in the same section of the river, but their preference for shallower and slower water isolated X. pygmaeus from them.

[3] Its preferred habitat is characterized by soft bottoms and large stands of submerged plants[3] such as Sagittaria.

[9] Xiphophorus pygmaeus breeds sparsely, with a female rarely giving birth to more than 10 fry at once.