Studies of the genera Sitobion, Uroleucon and Aphis (Hemiptera, Aphididae) known in South America, based on material preserved in the aphid collection of the University of Leon, are presented: “The genus Sitobion (Aphididae, Aphidinae, Macrosiphini) in South America, with new species, relocations of species and a hypothesis on the origin of the genus”, “The genus Uroleucon (Aphididae, Aphidinae, Macrosiphini) in Argentina and Chile, with taxonomic, bionomic and distribution novelties” and “Twelve Aphis new species (Aphididae, Aphidinae, Aphidini, Aphidina) in Southern South America, with an identification key for the Aphis known species in Argentina and Chile”. Regarding the genus Sitobion: (1) the difficulty of separating Nº 6: 1-70. 30-IX-2024 - Tres estudios taxonómicos sobre pulgones (Hemiptera, Aphididae) de América del Sur Suplementos del Boletín de la Asociación Española de Entomología - Nieto Nafría, Ortego & Mier Duranteit with certainty from Macrosiphum is explained; (2) the combination Sitobion edrossi n. comb. is established; (3) Sitobion pallidissimum n. sp. and Sitobion raukauae n. sp. are describes, respectively from apterous viviparous females collected on Dunalia spinosa (Solanaceae) in Peru, and from apterous and alate viviparae collected on Raukaua laetevirens (Araliaceae) inChile, and the characteristics that allow their differentiation from each other, from S. edrossi and from the five Sitobion species introduced in South America are provided; (4) the taxonomic situation of Macrosiphum salviae, Sitobion mesosphaeri, S. cuscutae and S. martorelli is presented, the first of them must remain in the genus Macrosiphum and the other three must be reintegrated into it; (5) a hypothesis of the Gondwanan origin of the genus Sitobion is presented, taking into account the distribution in the world of the known Sitobion species, with an unpublished contribution from Roger L. Blackman, who must be associated with the authorship of the hypothesis. Regarding the genus Uroleucon: (6) Uroleucon latteuri n. sp. and Uroleucon vondohlenae n. sp. are established, respectively from apterous viviparous females collected on Mutisia decurrens (Asteraceae) in the Argentine province of Río Negro, and from apterous and alate viviparous females collected on Tessaria absinthioides (Asteraceae) in the Argentine province of Salta, the Chilean region of Arica and Parinacota and the Peruvian department of Arequipa; (7) the synonymy between Nietonafriella syn. n. and Lambersius is established, and its type species N. euclypeata is transferred to Uroleucon: Uroleucon euclypeatum comb. n.; (8) the subjective synonymy between Uroleucon ambrosiae [valid name] and Uroleucon lizerianum is recognized, and the classification of Uroleucon essigi in the subgenus Lambersius is corroborated, both presented in 1998 by R. C. Z. Carvalho and collaborators; (9) the identity of Uroleucon littorale is discussed, and the taxonomic position from some authors of Uroleucon pseudomuermosum and U. muermosum is commented; (10) knowledge of the intraspecific variability of Uroleucon essigi, U. pseudomuermosum and U. tessariae is improved; (11) several new host plant relationships with U. ambrosiae, U. bereticum, U. capitophoroides, U. erigeronense, U. essigi, U. euclypeatum, U. gochnatiae, U. macolai, U payuniense, U. riojanum, U. sonchi and U. tucumani, are presented; (12) Uroleucon jaceae is recorded for the first time in Chile, and Uroleucon compositae is recorded for the first time since 1939 in Argentina; (13) U. aeneum, U. ambrosiae, U. bereticum, U. erigeronense, U. essigi, U. euclypeatum, U. gochnatiae, U. jaceae, U. mendocinum, U. payuniense, U. riojanum, U. sonchi and U. tessariae are recorded for first time in several Argentine provinces; and (14) an identification key of the species of the genus Uroleucon known in South America is presented. Regarding the genus Aphis: (15) Twelve species, classified into the nominotypical subgenus, are described: Aphis aguadorum n. sp. from apterous and alate viviparous females collected on Amaranthus quitensis (Amaranthaceae) in Argentina, Aphis barbagalloi n. sp. from apterous and alate viviparae collected on several Oenothera (Onagraceae) species in Argentina and Chile, Aphis binazzii n. sp. from apterous and alate viviparae collected on Salix humboldtiana (Salicaceae) in Chile, Aphis cordillerana n. sp. from apterous viviparae collected on an unidentified Lamiaceae species in Chile, Aphis exigua n. sp. from apterous viviparae collected on an unidentified plant in Chile, Aphis iadizai n. sp. from apterous and alale viviparae collected on Solanum tripartitum (Solanaceae) in Argentina and on Solanum crispum in Chile, Aphis insignis n. sp. from apterous and alate viviparae collected on Ribes magellanicum (Grossulariaceae) in Argentina and Chile, Aphis interposita n. sp. from apterous and alate viviparae also collected on Ribes magellanicum (Grossulariaceae) in Argentina, Aphis monninae n. sp. From apterous and alate viviparae collected on Monnina dictyocarpa (Polygalaceae) in Argentina, Aphis nickeli n. sp. from apterous and alate viviparae, oviparous females and males collected on several Acaena (Rosaceae) species in Argentina and Chile, Aphis salviaevora n. sp. from apterous and alate viviparae and oviparous females collected on Salvia cuspidata subsp. Gilliesii (Lamiaceae) in Argentina, and Aphis stekolshchikovi n. sp. From apterous and alate viviparae and oviparous females collected on Azorella trifurcata and Foeniculum vulgare (Apiaceae) inArgentina; (16) keys to apterous viviparous females of the Aphis species known living on the plant genera that hosting each new species are presented following the corresponding keys by Blackman and Eastop in Aphids on World’s Plants (2022); (17) a key to the apterous viviparous females of the 93 species of Aphis (subgenera Aphis, Bursaphis and Toxoptera) that have been cited in Argentina or Chile, is presented.