This investigation of Calicotyle affinis is the first ultrastructural description of the male reproductive system of a monocotylid monogenean. It has revealed numerous characteristics which are either new or clarify those determined from previous light microscopical observations on the morphology of Calicotyle spp. These include numerous individual testes, each surrounded by its own basal lamina; an ejaculatory bulb delimited by a basal lamina surrounded by two independent internal and external bands of muscles; a bipartite, but not fully divided, muscular chamber (an internal seminal vesicle) in the proximal region of the ejaculatory bulb; well-developed unicellular prostatic glands localised outside the ejaculatory bulb, the cytoplasmic extensions of which pass into the ejaculatory bulb, to form the ducts which open into the lumen of ejaculatory duct; and a male copulatory organ in the form of a coiled penis tube containing throughout its lumen a long, hollow stylet. The penis tube appears to be a modified prolongation of the ejaculatory duct and is supported by a basal lamina and muscle layers. The stylet wall consists of a homogenous, dense material with a narrow, denser inner lining; it is funnel-shaped proximally but narrows imperceptibly towards the distal end of the penis tube, and appears to be a derivative of the basal laminae of the ejaculatory bulb and distal ejaculatory duct. A comparative analysis of the nature of the stylet in relation to those found in free-living platyhelminths forms part of a search for characters useful for understanding monocotylid origins and relationships.