A new quill mite species of the family Syringophilidae (Acariformes: Prostigmata: Cheyletoidea) is described from Phimosus infuscatus (Lichtenstein) (Pelecaniformes: Threskiornithidae) in Argentina. Selenonycha insperata n. sp. differs from other species of the genus Selenonycha Kethley, 1970 by the presence of wing-like cuticular projections of coxal fields III–IV situated in front of trochanters (vs absence). An unexpected finding of this species on a bird of the family Threskiornithidae (Pelecaniformes) is discussed as an example of host shift. Additionally, a key to the species of the genus is provided.