Pseudocycnus appendiculatusHeller is described and recorded, for the first time from New Zealand, onThunnus alalunga(Bonnaterre, 1788).Pseudocycnus spinosusPearse, 1951 andP. thynnusBrandes, 1955 are thought to be synonyms of P.appendiculatus. Other species previously placed in the genus are discussed and the action of Yamaguti (1963) in removing them to other genera supported. It is suggested that the family Pseudocycnidae Yamaguti, 1963 cannot be differentiated from the family Dichelesthiidae Bassett‐Smith, 1898.