The pathogenesis of two histozoic myxosporidans, Henneguya sebasta and Kudoa clupeidae, was studied from the Pacific rockfish, Sebastes paucispinis. Infection of the bulbus and truncus arteriosus with H. sebasta was observed in 30 of 100 fish. The parasite metastasized throughout the tissue causing necrosis, hyperplasia and hypertrophy of the connective tissue and smooth muscle. K. clupeidae, found in 37 of 100 fish, caused necrosis and atrophy of host skeletal muscle. The unattractive appearance of Kudoa cysts in rockfish filets detracted from their commercial value.