AbstractThis study deals with two species of the genus Phtheiropoios, redescribing P. tucumanus Cicchino, 1990, previously known from five specimens collected on Ctenomys tucumanus Thomas, 1900, and describing P. inaequalissp. nov. from C. tuconax Thomas, 1925, both from different localities in Tucumán Province, Argentina. Diagnostic features for the new species include proportion of the male forficula, male external genitalia, and counts of setae and body measurements in both sexes. Brief comments on the morphological affinities of these species with other species of Phtheiropoios, and a key to males of all species of the genus Phtheiropoios known to parasitize rodents of the genus Ctenomys, are included.