The swordtail, Xiphophorus clemenciae (Poeciliidae), has been considered a species of special concern because of its apparent limited range. Although described in 1959, it is officially still known only from three locations in the Rio Coatzacoalcos basin, Mexico. Zoogeographic studies have now shown that this species is widespread and abundant but restricted to the uplands of the Rio Coatzacolacos basin where it replaces in many areas the common swordtail, X. helleri. Two new swordtail taxa, X. mixei and X. monticolus, are described from headwater streams of the Rio Jaltepec, a major Rio Coatzacoalcos tributary, Oaxaca, Mexico. The new forms are sympatric in part of their range and replace both X. clemenciae and X. helleri. Morphometric and molecular analyses revealed that X. clemenciae and the two new species constitute a monophyletic clade that exhibits a closer evolutionary affinity to the “northern” swordtails and the “platyfish” group of the genus rather than to X. helleri and the other “southern” swordtails. The evolutionary relationships of these taxa are discussed.