AbstractRecent studies based on morphological and molecular data provide a new perspective concerning taxonomic aspects of the brachyuran family Mithracidae. These studies proposed a series of nominal changes and indicated that the family is actually represented by a different number and representatives of genera than previously thought. Here, we provide a comparative description of the ultrastructure of spermatozoa and spermatophores of some species of Mithracidae in a phylogenetic context. The ultrastructure of the spermatozoa and spermatophore was observed by scanning and transmission electron microscopy. The most informative morphological characters analysed were thickness of the operculum, shape of the perforatorial chamber and shape and thickness of the inner acrosomal zone. As a framework, we used a topology based on a phylogenetic analysis using mitochondrial data obtained here and from previous studies. Our results indicate that closely related species share a series of morphological characteristics of the spermatozoa. A thick operculum, for example, is a feature observed in species of the generaAmphithrax,Teleophrys, andOmalacanthain contrast to the slender operculum observed inMithraculusandMithrax.AmphithraxandTeleophryshave a rhomboid perforatorial chamber, whileMithraculus,Mithrax, andOmalacanthashow a wider, deltoid morphology. Furthermore, our results are in agreement with recently proposed taxonomic changes including the separation of the generaMithrax(previouslyDamithrax),Amphithrax(previouslyMithrax) andMithraculus, and the synonymy ofMithrax caribbaeuswithMithrax hispidus. Overall, the spermiotaxonomy of these species of Mithracidae represent a novel set of data that corroborates the most recent taxonomic revision of the family and can be used in future taxonomic and phylogenetic studies within this family.