Diplozoon is known to display an exceptional biology of reproduction: the hermaphroditic adults are permanently fused together and their genital ducts communicate. In contrast to all other polyopisthocotylean monogeneans in which the spermatozoa show an homogeneous biflagellate structure, the spermatozoon of Diplozoon is aflagellate. It is filiform, and composed of a cytoplasmic region and a nuclear region. The cytoplasmic region exhibits mitochondria, a well-developed smooth endoplasmic reticulum, and up to 450 longitudinal singlet microtubules. The microtubules show links between them; seen in cross section, they are arranged as rows or polygons. The spermatozoon nuclear region contains the nucleus surrounded by cortical longitudinal microtubules. The spermiogenesis shows no zone of differentiation, a typical structure found in all other parasitic Platyhelminthes. Diplozoon is the first case of aflagellate spermatozoon found in the parasitic Platyhelminthes. The atypical sperm structure is not linked with phylogeny, but is well correlated with the atypical biology of reproduction.