During health surveys of 7- to 9-week-old goldfish (Carassius auratus L.) fingerlings in a fish farm near Budapest, Hungary, myxosporean plasmodia were observed on the fins. Plasmodia were most frequently found at the intersegmental joints of the finrays. Spores of Myxobolus diversus Nie et Li, 1973 known from China, were detectable in the mature plasmodia located within the lumen, and less often on the surface, of the cartilaginous finrays. The external wall of the plasmodia was constituted by a capsule formed from a collagenous material identical with the cartilaginous substance of the finrays. Matured plasmodia were filled by spores of 12-14 x 8-9.5 microm in size. The relatively small plasmodia caused only small deformities on the fins. Their importance is, however, not negligible, as in an ornamental fish such as the goldfish even a minor damage of the fins causes a loss of value. Besides a report on the first European occurrence and pathological aspects, a redescription of this parasite of Far-Eastern origin is given.