Myxobolus cyprini has, until now, been considered an ‘organo-cosmopolitan’ parasite of the common carp (Cyprinus carpio) and less frequently of other carps, producing spores in various organs in small plasmodia and possible in cysts. The present observations of naturally infected common carp fry and two-summer carp have revealed that M. cyprini is a specific muscle parasite, developing intracellularly in the muscle fibres of the skeletal muscle. The sarcoplasm of the infected muscle fibres is filled with developmental stages, followed by spores of M. cyprini, which are held together in a 1–1·5 mm long pseudocyst by the sarcolemma of the muscle fibre. After maturation of the spores and disintegration of the pseudocysts the spores are transported in the bloodstream to different parts of the body where they are retained in the capillaries.