The spermatozoal ultrastructure was investigated in male penaeid shrimp Artemesia longinaris from Mar del Plata (Buenos Aires Province, Argentina) fishing grounds. The spermatozoon of A. longinaris is a typical dendrobranchiate sperm, basically consisting of a subspheroidal main body and a long spike. The complex acrosomal structure is comprised of the acrosomal vesicle, formed by the acrosomal cap and spike, and the subacrosomal substance. A central, rounded protuberance arises from the concave side of the cap opposite the spike. As inferred from longitudinal and transverse sections, the spike is filled with densely packed longitudinal filaments, which are coiled and form a loose helix. The occurrence of an apomorphic protuberance at the base of the acrosomal cap in A. longinaris, Parapenaeus longirostris, and Penaeopsis serrata justifies the grouping of these three genera into the tribe Parapeneini. The filamentous inner structure of the spike appears to indicate a closer proximity between A. longinaris and P. serrata than between either of them and Parapenaeus longirostris.