Development of a new myxozoan parasite Tetracapsula bryozoides gen. n. et sp. n. in the coelomic cavities of Cris-latella mucedo Cuvier is described. Uninucleate proliferative cells are formed within well-defined sacs, the wall of which is one cell thick. The sacs, of different sizes according to age, are free floating and are conspicuously moved about within the coelomic fluid by the ciliary movements of the host. Division of the proliferative cells produces spherical cells of different sizes with nuclei of commensurate size. The largest cells enter sporogony by dividing into ten cells. Four of these become capsulogenic cells arranged as an anterior group, each giving rise to a spherical polar capsule containing a polar filament, possibly without prior formation of an external tube or, at most, very transient formation of these. Four valvogenic cells enclose the two sporoplasms and overlie the capsulogenic cells except at the points of exit of the polar filaments from the polar capsules. The two uninucleate sporoplasms are packed with endoplasmic reticulum, numerous mitochondria with tubular cristae and sporoplasmosomes which are distributed peripherally. Both sporoplasms produce secondary cells. Typical myxosporean features of the wall cells of the sac and all stages within the sac are: nuclei with granular nucleoplasm and prominent nucleolus, gap junctions between cells consisting of thickened membranes with cross connections, and haplosporosomes. A new genus is established for the parasite, defined as having development limited to uninucleate pseudoplasmodia within a sac of parasite origin, each uninucleate sporogonie stage giving rise to one spore with tetraradial symmetry, composed of four shell valves, four anterior polar capsules and two uninucleate sporoplasms with secondary cells. No plasmodia are formed. The genus is placed within the order Multivalvuli-da, in a new family Saccosporidae, defined as having development within a sac of parasite origin and sporogony without external tube or microtubules during polar capsule formation.