The developmental cycle of stages ofSphaerospora renicola, proliferating in the walls of the swimbladder in carp fry and responsible for swimbladder inflammation (=SBI stages) is described. The initial stage, a primary cell containing two secondary cells, grows in size while the secondary cells divide and produce two tertiary cells each by endogenous division. After a supposed disintegration of the grown primary cell, the cell triplets are released to repeat the cycle. The cell divisions are asynchronous and the development of the whole formation may be quite irregular. The cytology of these stages is closely reminiscent of the blood stages ofS. renicola and also shares common features with the parasite''s sporogonic stages. The SBI stages show typical features of myxosporean organisation, i.e., a cell-within-cell arrangement in which the primary cell contains secondary cell(s) in membrane-bound vacuoles and many free ribosomes, in the absence of centrioles. Othersalient features of SBI stages are partitions in the nuclei; a small amount of cytoplasm in the early secondary cells; distinct, age-dependent changes in morphology of the nucleus; and atypically shaped Golgi bodies in the cytoplasm or integrated with the nuclear envelope. The cytological features and significance of swimbladder stages are discussed in detail.