Abstract
A new species of flea (Siphonaptera: Rhopalopsyllidae) is described from sigmodontine rodents collected during a survey of small mammals in northwestern Argentina. The new species belongs to the subgenus Tetrapsyllus and can be distinguished from all species of the subgenus by characteristics of the modified abdominal segments and genitalia. A phylogenetic analysis of the genus Tetrapsyllus using morphological characters is presented and the analysis supports the erection of this new Tetrapsyllus species in the subgenus Tetrapsyllus. The unique occurrence of a sinus in the caudal margin of the sixth sternite of females that is present only in the subgenus Tetrapsyllus (also unique across the Order Siphonaptera) is provided with a discussion of how this adaptation is mirrored in the parallel development of a spur on the sclerotized inner tube of males. Additional host and locality data are recorded for Tetrapsyllus (P.) bleptus. A key to the genus Tetrapsyllus is provided. The increased diversity of flea fauna bordering a known plague endemic area in northwestern Argentina may prove useful in implementing sustainable control measures in the future.