Sphaerospora molnari sp.nov. is described from the gills, skin and nasal pits of yearling carp (Cyprinus carpio L.) from ponds in the South Bohemian pond region of Czechoslovakia and one infected specimen from the river Danube in Bulgaria. Small vegetative stages, often massively pervading the infected tissue, produce 1 spore each. Spores are sub-spherical, 10·3 × 10·5 μm in size, with almost spherical polar capsules of equal size. The polar filaments are wound in wide coils that are almost parallel with the longitudinal axis of the spore. There are 2 uninucleate sporoplasm cells. Developmental stages and spores are also found in circulating blood. In the gills, S. molnari invades stratified epithelium of gill filaments and secondary lamellae causing branchial lesions and must be regarded as a serious pathogen of yearling carp. A new name, Sphaerospora chinensis sp.nov. is proposed for S. branchialis Lee & Nie in Chen (1973), pre-occupied by S. branchialis Razmashkin and Skrip-chenko, 1967.