Opegaster ouemoensis sp. n. is described from Periophthalmus argentilineatus Valenciennes (Gobiidae). Distinctive features included the weak or undetectable papillae of the ventral sucker and the small, but distinct cirrus-sac. The new species is compared with 25 marine species of Opegaster for which a table of measurements and ratios is presented. The new combination Opegaster queenslandicus (Aken'Ova, 2007) (originally in Opecoelus) is formed. Fifteen mudskippers were intensively examined for parasites; larval anisakid nematodes and acanthocephalans were found, but no monogeneans, cestodes, copepods, isopods, hirudineans or adult nematodes. A brief summary of the helminth parasites of mudskippers is included.