The spermatozoon of an undescribed species of Dolichodasys (Cephalodasyidae) from the Pacific coast of Panama was studied at structural and ultrastructural levels. Under optical microscopy, it appears as a short and wide cell with pointed extremities but without a flagellum. The cell body is made up of two well distinct regions: an anterior region with a homogeneous appearance, and a posterior region containing an evident rod-like nucleus. Under TEM, a peripheral layer of microtubules densely arranged extends for the whole cell length. In the anterior cell region, microtubules surround many tubular cisternae of smooth endoplasmic reticulum (SER), and a thin layer of vesicles with a probable acrosomal function lies just beneath the plasma membrane. The rod-shaped nucleus fills up the posterior cell region and forms a pouch that hosts a single large, irregular mitochondrial mass. A hypothesis about the motility of this aflagellate cell is advanced, on the basis of the coexistence of singlet microtubules and SER. The general architecture of Dolichodasys sp. spermatozoon departs from the Macrodasyida sperm basic model, consisting of a filiform cell with a corkscrew-shaped acrosome, a spring-shaped nucleus surrounding a mitochondrial axis and an ordinary flagellum. The unusual morphology of the Dolichodasys sperm seems to be unique in the family Cephalodasyidae: the data available for 6 species belonging to the other 4 genera of the family report spermatozoa perfectly matching the basic sperm plan of the Macrodasyida. A sister-taxon relationship between Dolichodasys and Cephalodasys, two genera drastically different in sperm shape, emerged from recent phylogenetic molecular studies, but it needs confirmation due to the still limited number of molecular data and the likely polyphyletic nature of the family Cephalodasyidae.