Sphaerospora renicola n.sp. is a common parasite of carp in Czechoslovakia. Its life cycle involves intracellular stages in the epithelial cells of renal tubuli and trophozoite stages proliferating in the tubular lumen, transforming ultimately into pansporoblasts, each having one pansporoblast nucleus and producing two spores. The spores are almost globular with an average size of 7.3×7.2 , with polar capsules of equal size, and may have two slightly protruding tubercles on their shell valves. Differential diagnosis from otherSphaerospora species infecting carp, as well as fromMitraspora cyprini Fujita, is made. Intracellular stages ofS. renicola cause swelling and hyperplasia of the epithelium in renal tubuli followed by dystrophic changes. Accumulation of developmental stages in the tubular lumen provokes pronounced regressive changes of the epithelium, which may be followed by necrosis.