AbstractDue to its biological and systematic importance, the morphology and function of the male copulatory apparatus of Haminoea navicula, a Cephalaspidea gastropod mollusk, was investigated by light and electron microscopy. These systems are poorly understood in haminoids, but are often used in the taxonomy of the genus. In H. navicula, the male copulatory apparatus comprises the penis within a penial sheath, a seminal duct and the prostate with two lobes. The penis is a muscular structure with a tip covered by spikes formed by muscular cells. The penial sheath consists of muscular tissue folds lined by an epithelium. Below this epithelium, polysaccharide‐secreting cells and pigment cells were observed. A large number of vacuolar cells were found below the ciliated epithelium of the seminal duct. The proximal lobe of the prostate was formed by tubules that could be divided in basal, middle and apical zones, containing cells that secrete polysaccharides and proteins. The tubules of the prostate distal lobe contained a single type of secretory cells with vesicles that were stained by histochemical techniques for detection of polysaccharides and proteins. Ciliated cells were present along the tubules in both lobes of the prostate. This study revealed a complex prostate with five types of secretory cells, which secrete substances that should be involved in the maintenance and eventually also in the maturation of spermatozoa. A comparison with previous publications, shows that the male copulatory apparatus can differ substantially among cephalaspideans, even between H. navicula and non‐European species attributed to this genus.