AbstractAphid species colonising ferns belong to the subfamily Aphidinae (Hemiptera: Aphididae) and the majority of these to the tribe Macrosiphini. A new genus in this tribe and its type species:Gibbomyzus pteridophytorumnew genus,new species, are established. Apterous and alate viviparous females are described from specimens collected onBlechnum buchtieniiRosenstock (Blechnaceae) in Costa Rica and onPteridium aquilinum(Linnaeus) Kuhn (Dennstaedtiaceae) and an unidentified fern in Mexico. The taxonomic validity of the two new taxa is discussed based on morphological and molecular data. Morphologically, the new genus is compared with genera with swollen siphunculi recorded in the New World, and also with genera living on ferns anywhere in the world. The identification key by Blackman and Eastop to aphids living on ferns is modified. Molecular analyses were carried out on the sequences of a fragment of the mitochondrial gene encoding for cytochromecoxidase subunit 1 and of a fragment of the nuclear gene encoding elongation factor-1 alpha. In both analyses,G. pteridophytorumnewspeciessequences showed considerable divergence (∼4% or more) from those of 23 other species from diverse genera of Macrosiphini, supporting the conclusions of the morphological study and justifying the establishment of the new genus.