Acuaria paraguayensis n. sp. from Sirystes sibilator (Aves: Tyrannidae) in Paraguay and a redescription of A. mamillaris (Molin, 1860) from Cyanocorax cayanus (Corvidae) in Brazil, with a key to the species of Acuaria Bremser, 1811 (Nematoda: Acuariidae) in the New World.

Acuaria paraguayensis n. sp. is described on the basis of specimens from Sirystes sibilator (Vieillot) (Aves: Passeriformes, Tyrannidae) in Paraguay. In addition, A. mamillaris (Molin, 1860) from Cyanocorax cayanus (L.) (Corvidae) in Brazil is redescribed on the basis of its type-series from the collection of the Naturhistorisches Museum, Vienna. A review of the species of Acuaria Bremser, 1811 in the New World is presented. Currently, 16 species belong to this genus, which are mostly parasitic in passeriform birds (one record in piciform birds). An identification key to the species of Acuaria occurring in the New World is presented. Acuaria multispinosa (Vigueras, 1938) originally described from Botaurus lentiginosus (Rackett) (Ardeidae) in Cuba, also known from various herons (Ciconiiformes, Ardeidae) in southern states of the USA, does not correspond to the generic diagnosis of Acuaria and is considered a species incertae sedis. Acuaria gracilis var. sturni Boyd, 1951 is elevated to full species rank as Acuaria sturni Boyd, 1951.