New caligiform Copepod (Siphonostomatoida: Dissonidae) parasitic on Seriola hippos from western Australian waters, with new records and orphological variation for Dissonus nudiventris and Dissonus similis, and an updated key to the species of Dissonus.

Dissonus hoi n. sp. is described from female and male specimens collected from the nasal cavities of a Samson Fish (Seriola hippos Günther) captured off Rottnest Island, Western Australia. Adult female D. hoi are distinguished from their congeners by possessing the following combination of characters: (1) a quadrangular genital complex, (2) ventral spines on the genital complex, (3) pair of postantennal processes, (4) a sternal furca, (5) 1-segmented abdomen, and (6) convoluted, uniseriate egg strings. New records and intraspecific morphological variation are also given for Dissonus nudiventris collected from Australia, and Dissonus similis collected from the tropical western Pacific. A key to the valid species of Dissonus is provided.