The study of the gill infection of rockfishes, Sebastes spp. by monogenetic trematodes conducted during March 2002 to June 2002 are presented. Three species are reported krom examination of 23 rockfishes collected from Oregon Coast Aquarium and Newport fish market, Oregon. Ten rockishes belonging to three species showed an infection rate of 43.5%. A total of 135 monogenetic trematodes was found in the gills of Sebastes spp. Of 135 worms recovered from the gills of 4 harbor rockfishes, two were Sebastes maliger, 14 S. melanops, 110 S. pinniger, and 9 S. crameri. Prevalence and intensity in Microcotyle seba.fis were 11.9％ and 4.0, and for Trochopus trituba they were 81.5% and 55.0, and for Trochopus australis, they were 6.7% and 2.3, respectively. These worms are mainly found firmly attached to the primary lamellae of the gills of the rockfishes.