We present here a description of the spermatozoon of Rhopalura littoralis, the first fine-structural study of an orthonectid sperm. The spermatozoon contains a slightly elongate nucleus and two centrioles orientated along the longitudinal axis of the sperm. The proximal centriole bears one rootlet. A single mitochondrion is present in the mid-piece region. An acrosome is absent. The sperm tail is a simple flagellum with 9+2 structure. We consider the orthonectan spermatozoon to be closer in structure to those of Porifera, Cnidaria, and Annelida, than to Aschelminthes and Platyhelminthes, to which orthonectids have previously been allied.