A single specimen of a strongly transformed, endoparasitic organism was found on the gymnosomatous pteropod Notobranchaea macdonaldi Pelseneer morpha pelseneeri Pruvot-Fol off Delaware Bay in the north-western Atlantic Basin. The parasite is described as Pteroxena papillifera n. gen., n. sp., and is provisionally attributed to the subclass Copepoda of the Crustacea.