[[Jenynsia Günther]]

The genus Jenynsia Günther is comprised of 11 species of small viviparous fishes, which are diagnosed by the possession of tricuspidate teeth in the outer mandibular series in adults, and an unscaled tubular gonopodium formed principally by anal-fin rays 3, 6, and 7 (Parenti, 1981). This genus is distributed in the Río de la Plata basin, coastal Atlantic drainages from Rio de Janeiro in Brazil to Río Negro Province in Argentina, and in the endorheic Río Salí -Dulce basin, in northwestern Argentina.

In his phylogenetic analysis of the family Anablepidae, Ghedotti (1998) recognized two clades within Jenynsia, which were formally recognized by him as the subgenera Plesiojenynsia and Jenynsia. Two new species have recently been described from southern Brazil, Jenynsia weitzmani Ghedotti, Downing-Meisner & Lucinda and J. onca Lucinda, Reis & Quevedo. According to the phylogenetic framework put forward by Ghedotti (1998); these species belong in the monophyletic subgenera Plesiojenynsia and Jenynsia, respectively.

In Argentina four species of Jenynsia are represented, all in the subgenus Jenynsia. Of these, only J. pygogramma Boulenger and J. maculata Regan have their type localities within the country. In the upper Río Salí basin, in Tucumân, two species of this genus were cited as Jenynsia sp. A and Jenynsia sp. B (Butí & Miquelarena, 1995). The first of these species corresponds to the broadly distributed Jenynsia multidentata (Jenyns), and the latter is the new species herein described. The purpose of this paper is to describe Jenynsia tucumana n. sp. from the Río Vípos, Tucumân, Argentina, and to discuss its phylogenetic relationships.