Abstract.Laboratory examinations of pond‐reared common carp,Cyprinus carpioL., revealed a close correlation between the prevalence of swimbladder inflammation (SBI), renal sphaerosporosis and infection by C‐blood‐protozoan among the carp fry. In impression smears as well as light and electron microscopic preparations we detected developmental stages of intercellular protozoa, mainly in the loose fibrous tissue of the swimbladder. The parasites multiplied by internal budding so that 20–46 secondary cells were formed in each primary cell and two tertiary cells were formed in each secondary cell. The final stage of development was a unit consisting of a secondary cell enclosing two tertiary cells, i.e. a so‐called triple formation, which bore a close resemblance to the early sporogonic stages of the renal sphaerosporanSphaerospora angulataFujita, 1912. Certain morphological similarities and the frequent simultaneous presence of the swimbladder protozoan, C‐bloodprotozoan, andS. angulatain hosts with clinical SBI have led us to postulate that the former two parasites could represent the hitherto unknown presporogonic stages ofS. angulata.In view of the pathological changes caused by the parasites in the hosts with clinical SBI, and negative bacteriological and virological findings we have postulated that the swimbladder protozoan is the primary cause of SBI