Original description as Xiphophorus pygmaeus nigrensis:
- Rosen, Donn E. 1960. "Middle-American poeciliid fishes of the genus Xiphophorus". Bulletin of the Florida State Museum, Biological Sciences. 5(4):57-242 (ffm00047)
Conservation: Xiphophorus nigrensis is evaluated by the international union for the conservation of nature in the iucn red list of threatened species as (DD) data deficient (2018). The habitat of Xiphophorus nigrensis is a small river (about 5 m wide) with a large flow that is born at "Nacimiento de Tamuin" (previously known as Nacimiento de Taninul) spring and runs partly inside pristine terrains owned by the local society of cane growers, whom allow access to the spring some days a week for a fee. the rest of the river before it gets to the town of Tamuin [San Luis Potosi] and merges the Tampaon River runs mostly through private cattle growing lands for about ten kilometers. Apparently due to pollution, Xiphophorus nigrensis is just found in about half of this length. The conservation status in accordance to the current IUCN rules should be considered vulnerable.