AbstractThe hermaphroditic marine snail species Haminella solitaria was formerly included in the genus Haminoea, but it was recently assigned to the genus Haminella. The copulatory apparatus in H. solitaria was investigated by light and transmission electron microscopy to obtain additional information about this apparatus in cephalaspidean gastropods and to evaluate the taxonomic relevance of its morphofunctional features in the framework of a new phylogenetic tree of the family Haminoeidae. The copulatory apparatus in H. solitaria consisted of the atrium with a muscular wall and papilla, a seminal duct, and a single‐lobed prostate. Epithelial and subepithelial secretory cells were detected in the proximal and middle region of the atrium wall, and a third type of secretory cell occurred in the distal region of the muscular papilla. The seminal duct was lined by ciliated cells and its muscular wall included some vacuolar cells. The prostate in H. solitaria consisted of lateral pouches surrounding a large central lumen that was filled with spermatozoa. A single type of secretory cell intermingled with ciliated cells formed the epithelium of the prostate. A histological comparison between the copulatory apparatus in H. solitaria and Haminoea navicula revealed substantial differences that support the placement of these two species in different genera, as established by recent molecular studies.