The structure of the spermatozoon and spermatogenesis ofDiscinisca tenuis(Inarticulata) andKraussina rubra(Articulata) are described by transmission electron microscopy. The spermatozoon ofD. tenuishas a similar morphology to that of other inarticulate brachiopods. The head (about 1.9 μm long) is composed of a nucleus which is invaginated anteriorly and posteriorly, the anterior invagination partially housing the pyriform acrosome. The acrosome is differentiated internally and is separated from the nucleus by subacrosomal material. The mid-piece is composed of a ring of four or five spherical mitochondria which surround the proximal and distal centrioles which are arranged orthogonally. By contrast the spermatozoon ofK. rubrahas a head which is 2 μm long × 1.1 μm in diameter. The nucleus which is rounded anteriorly is capped by a small (0.5 μm diameter) disc-shaped acrosome. The mid-piece has a single ring-shaped mitochondrion which surrounds the centrioles which are almost in line with each other. In both species, during spermatogenesis, each spermatogonium and spermatocyte develops a flagellum which appears to be absorbed prior to cell division. InD. tenuis, proacrosomal vesicles form within spermatogonia whereas acrosome formation inK. rubradoes not commence until spermiogenesis. In the early spermatid of both species a single acrosomal vesicle forms in the presumptive posterior of the cell and during spermiogenesis the developing acrosome migrates anteriorly where it assumes its mature form.