The ultrastructure of spermiogenesis and the mature spermatozoa has been studied in Eusepia officinalis, Loligo forbesi and Alloteuthis subulata, involving the application of the freeze-etching technique to the morphology of the mature gamete. The spherical spermatid cells each become modified to give an elongate spermatozoon. The gametes are distinguished from other metazoan spermatozoa by the shape of the feebly bent head, the laterally placed centriole to one side of the nuclear base, and the separate axoneme and mitochondrial pocket. The membrane organization in the mid-piece region is described and the structure of the modification of the plasmalemma at this point elucidated. There is a single centriole within the centriolar pit and the axoneme has a 9+9+2 structure comparable with that of other cephalopod spermatozoa. Despite the variation in tail length, the morphology is very similar in the three species examined. The mature nucleus exhibits a 'pseudolaminar' chromatin packing system in freeze-etched replicas.