The ultrastructure and cytochemical features of Sphaerospora testicularis found in the testes of cultured sea bass (Dicentrarchus labrax) are described. A hypothesis for the developmental cycle of this myxosporean in the lumen of seminiferous tubules is suggested. The initial stage, a primary cell (P), generates by endogenous division a secondary cell (S) which divides mitotically producing two S cells. The latter divide again by endogeny generating tertiary cells (T) within them. After mitotic division, T cells can be considered sporogonic, as they undergo meiosis and further differentiation giving rise to mature spores. Differences of this cycle to that of other sphaerospores are discussed. The S. testicularis stages show the typical features of myxosporean organization. Cytochemically, low levels of polysaccharides were detected in most stages, but lipidic vacuoles were relatively abundant. Desmosome-like junctions occur between capsulogenic cells, and there is a filamentous plug attachment of the polar capsule.