Two new species of myxosporeans (Myxosporea: Sinuolineidae), Myxoproteus cheni sp. n. and Sinuolinea mai sp. n., parasitized in marine fishes collected from the Yellow Sea, off the Qingdao coast of China, are described. Both species, including spores and plasmodia were found in the urinary bladder of the host, Thamnaconus septentrionalis Gunther, 1877. The diagnostic features of Myxoproteus cheni sp. n.: trophozoites monosporous or disporous; spore inversely pyramidal or triangular to subspherical with smooth surface, fine sutural line straight or slightly sinuous, 14.0±0.8 (12.8-15.3) x 12.8±1.3 (11.2-15.0) x 12.5±1.5 (11.2-15.0) µm in size; two spherical polar capsules located anteriorly and conspicuously separated from each other, 3.6±0.4 (3.2-4.2) µm in diameter; coelozic. Sinuolinea mai sp. n.: trophozoites disporous; spore body inversely pyramidal to spherical with smooth surface, sutural line highly sinuous, 22.4±0.5 (21.7-23.0) x 19.5±0.5 (18.8-20.2) µm in size; two spherical polar capsules located anteriorly and conspicuously separated from each other, 5.8±0.4 (5.0-6.3) µm in diameter; coelozic.