Summary The structure of the spermatozoon and spermatogenesis of Discinisca tenuis (Inarticulata) and Kraussina rubra (Articulata) are described by transmission electron microscopy. The spermatozoon of D. tenuis has 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 of K. rubra has 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 ea...