The sperm ultrastructure ofMeganyctiphanes norvegicawas examined with the aim of investigating phylogenetic relationships between Euphausiacea and Decapoda. A noticeable feature of the spermatozoon ofM. norvegicais the presence of a thick, two-layered capsule that is in close contact with the plasma membrane. The sperm ultrastructure in this species appears to be similar to that ofEuphausiasp., the only other euphausiid investigated to date. The sperm of both euphausiids resemble those of some species of decapods in that they have a loose central chromatin surrounded by the cytoplasmic ring and lack an acrosome, nuclear envelope and radial arms. These features are suggested to represent eucarid plesiomorphic characters.