Northwestern Argentina is an area of a high biodiversity. However, few fleas have been recorded parasitizing mammals. In the present study we report the results of surveys carried out in the provinces of Salta, Jujuy, Tucumán, and Catamarca. The following 9 flea species and subspecies were collected from 17 species of mammals: Craneopsylla minerva minerva (Rothschild, 1903) (Stephanocircidae); Neotyphloceras crassispina hemisus Jordan, 1936 (Ctenophthalmidae); Adoratopsylla (Tritopsylla) intermedia intermedia (Wagner, 1901) (Ctenophthalmidae); Tetrapsyllus (Phylliver) bleptus (Jordan and Rothschild, 1923) (Rhopalopsyllidae); Polygenis (Polygenis) acodontis (Jordan and Rothschild, 1923) (Rhopalopsyllidae); Phthiropsylla agenoris (Rothschild, 1904) (Malacopsyllidae); Malacopsylla grossiventris (Weyenbergh, 1879) (Malacopsyllidae); Pulex irritans Linnaeus, 1758 (Pulicidae); and Hectopsylla gracilis Mahnert, 1982 (Tungidae). Neotyphloceras crassispina hemisus, Ad. (T.) i. intermedia, Ph. agenoris, and H. gracilis are reported for the first time from northwestern Argentina, from specimens collected in the provinces of Jujuy, Salta, and Catamarca. The mammals Micoureus constantiae (Thomas, 1904); Thylamys cf. cinderella (Thomas, 1902); Thylamys sponsorius (Thomas, 1921) (Didelphimorphia); Akodon albiventer Thomas, 1897; Akodon caenosus Thomas, 1918; Calomys lepidus (Thomas, 1884); Eligmodontia hirtipes Thomas, 1902; Eligmodontia puerulus (Philippi, 1896); Phyllotis xanthopygus (Waterhouse, 1837); Tapecomys primus Anderson and Yates, 2000 (Rodentia); and Chaetophractus vellerosus (Gray, 1865) (Cingulata) have an increased number of parasite species known to them. Eighteen new parasite–host associations are also reported.