Xiphophorus nigrensis, the Panuco swordtail, is a species of fish in the family Poeciliidae[2] that is endemic to a small part of the Pánuco River basin in Mexico.

[1][3][4] Being a small swordtail, Xiphophorus nigrensis was originally considered a subspecies of X.

The males possess on their caudal fin a moderately to well-developed sword set off by a black line, from which the specific epithet nigrensis (niger meaning black and ensis sword in Latin) is derived.

Larger individuals are deeper-bodied and develop brilliant iridescent blue on their sides and flanks, which may somewhat conceal their midlateral black stripe.

[7] The species inhabits clear, deep, fast-flowing waters along undercut slopes of river and stream beds.