Ceratomyxa seriolae n. sp. and C. buri n. sp. (Myxozoa: Myxosporea) were found in the gall-bladder of cultured yellowtail Seriola quinqueradiata Temminck & Schlegel (Carangidae) in Japan. Mature spores of C. seriolae n. sp. were elongate and 6.5 (6.0-7.5) microm long and 33.7 (28.0-41.5) microm thick. Disporous plasmodia of C. seriolae n. sp., 40-100 microm in size, were amoeboid to spherical. C. buri n. sp. were elliptical with a flattened posterior end. 6.5 (5.5-7.5) microm long and 14.3 (11.0-16.5) microm thick. Spherical plasmodia of C. buri n. sp., 15-20 microm in diameter, were disporous. In periodical sampling of yellowtail bile from August, 1999 to February, 2000, the two new species of Ceratomyxa, as well as Myxobolus spirosulcatus Maeno, Sorimachi, Ogawa & Kearn 1995, first appeared in October, and the prevalences were very variable (20-100%) during the study period.