This study describes the sperm morphology of the mayfly Hexagenia (Pseudeatonica) albivitta (Ephemeroptera). Its spermatozoon measures approximately 30 μm of which 9 μm corresponds to the head. The head is composed of an approximately round acrosomal vesicle and a cylindrical nucleus. The nucleus has two concavities, one in the anterior tip, where the acrosomal vesicle is inserted and a deeper one at its base, where the flagellum components are inserted. The flagellum is composed of an axoneme, a mitochondrion and a dense rod adjacent to the mitochondrion. A centriolar adjunct is also observed surrounding the axoneme in the initial portion of the flagellum and extends along the flagellum for at least 2 μm, surrounding the axoneme in a half-moon shape. The axoneme is the longest component of the flagellum, and it follows the 9+9+0 pattern, with no central pair of microtubules. At the posterior region of the flagellum, the mitochondrion has a dumb-bell shape in cross sections that, together with the rectangular mitochondrial-associated rod, is responsible for the flattened shape of the flagellum. An internal membrane is observed surrounding both mitochondrion and its associated structure.962160166Baccetti, B., Insect sperm cells (1972) Advances in Insect Physiology, 9, pp. 315-397Baccetti, B., Dallai, R., Giusti, F., The spermatozoon of Arthropoda. VI Ephemeroptera (1969) Journal of Ultrastructure Research, 29, pp. 343-349Baccetti, B., Dallai, R., Pallini, V., Rosati, F., Afzelius, B.A., Protein of insect sperm mitochondrial crystals - Crystallomitin (1977) The Journal of Cell Biology, 73, pp. 594-600Barber-James, H.M., Gattlliat, J.L., Sartori, M., Hubbard, M.D., Global diversity of mayflies (Ephemeroptera, Insecta) in freshwater (2008) Hydrobiologia, 595, pp. 339-350Barber-James, H., Sartori, M., Gattolliat, J.-L., Webb, J., World checklist of freshwater Ephemeroptera species (2013) World Wide Web electronic publication, , http://fada.biodiversity.be/group/show/35, Available online at [17/June/2013]Blanke, A., Wipfler, B., Letsch, H., Koch, M., Beckmann, F., Beutel, R.G., Misof, B., Revival of Palaeoptera-head characters support a monophyletic origin of Odonata and Ephemeroptera (Insecta) (2012) Cladistics, 28, pp. 560-581Brito, P., Salles, F.F., Dolder, H., Characteristics of the male reproductive system and spermatozoa of Leptophlebiidae (Ephemeroptera) (2011) Neotropical Entomology, 40, pp. 103-107Brittain, J.E., Sartori, M., Encyclopedia of insects (2003) Ephemeroptera (Mayflies), pp. 373-380. , In: Resh, V. H. and Cardé, R. T. (Eds): Academic Press, AmsterdamDallai, R., Afzelius, B.A., Microtubular diversity in insect spermatozoa: Results obtained with a new fixative (1990) Journal of Structural Biology, 103, pp. 164-179Dallai, R., Afzelius, B.A., Sperm structure of Trichoptera. I. Integripalpia: Limnephiloidea (1993) International Journal of Insect Morphology and Embryology, 23, pp. 197-209Dallai, R., Beani, L., Kathirithamby, J., Lupetti, P., Afzelius, B.A., New findings on the sperm structure of Xenos vesparum (Rossi) (Strepsiptera, Insecta) (2003) Tissue and Cell, 35, pp. 19-27Dallai, R., Lupetti, P., Mencarelli, C., Unusual axonemes of Hexapod spermatozoa (2006) International Review of Cytology, 254, pp. 45-99Dallai, R., Lombardo, B.M., Lupetti, P., Sperm ultrastructure in Chironomoidea (Insecta, Diptera) (2007) Tissue and Cell, 39, pp. 179-194Fink, T.J., Yasui, L.S., Ultrastructure of the sperm of Dolania americana. Edmunds and Traver (Ephemeroptera: Behningiidae) (1988) International Journal of Insect Morphology and Embryology, 17, pp. 447-454Gaino, E., Mazzini, M., Spermatozoon of mayflies (Ephemeroptera): An ultrastructural approach (1991) Overview and Strategies of Ephemeroptera and Plecoptera, pp. 27-38. , In: Alba-Terzrcedor, J. and Sanchez-Ortega, A. (Eds): The Sandhill Crane Press, GainesvilleGaino, E., Mazzini, M., Aflagellate sperm in three species of Leptophlebiidae (Ephemeroptera) (1991) International Journal of Insect Morphology and Embryology, 20, pp. 119-125Gottardo, M., Mercati, D., Dallai, R., The spermatogenesis and sperm structure of Timema pappensis (Insecta: Phasmatodea) (2012) Zoomorphology, 131, pp. 209-223Grimm, V.R., Vergleichend-anatomische Untersuchung der abdominalen Muskulatur und der Gonoducte männlicher Ephemeroptera-Imagines (Insecta) (1985) Stuttgarter Beiträge zur Naturkunde (Serie A), 59, pp. 1-59Hennig, W., (1981) Insect Phylogeny, , John Wiley & Sons, New YorkJamieson, B.G.M., (2011) The Ultrastructure and Phylogeny of Insect Spermatozoa, p. 320. , Cambridge University Press, Cambridge, UKJamieson, B.G.M., Dallai, R., Afzelius, B.A., (1999) Insects: Their Spermatozoa and Phylogeny, , Science Publisher, Enfield, USAKluge, N.J., (2004) The Phylogenetic system of Ephemeroptera, p. 456. , Kluwer Academic, Dordrecht, The NetherlandsKristensen, N.P., Phylogeny of insect orders (1981) Annual Review of Entomology, 26, pp. 135-157Kukalová-Peck, J., Fossil history and the evolution of hexapod structures (1991) The Insects of Australia, 1, pp. 141-179. , In: Naumann, I. D. (Ed.):, 2nd edn, CSIRO, Melbourne University Press, Melbourne, AustraliaLino-Neto, J., Báo, S.N., Dolder, H., Sperm ultrastructure of the honey bee (Apis mellifera) (L) (Hymenoptera, Apidae) with emphasis on the nucleus-flagellum region (2000) Tissue and Cell, 32, pp. 322-327Lupetti, P., Mencarelli, C., Mercati, D., Gaino, E., Dallai, R., The spermatodesm of Cloeon dipterum (L.): Fine structure and sperm movement (2011) Tissue and Cell, 43, pp. 157-164Moreira, J., Brito, P., Mancini, K., Dolder, H., Lino-Neto, J., The descriptions of new microanatomical structures of the male reproductive system and sperm of Myschocyttarus cassununga (Hymenoptera: Vespidae) (2012) Micron, 43, pp. 292-297Ogden, T.H., Whiting, M.F., The problem with "the Paleoptera Problem": Sense and sensitivity (2003) Cladistics, 19, pp. 432-442Ogden, T.H., Gattolliat, J.L., Sartori, M., Staniczek, A.H., Soldán, T., Whiting, M.F., Towards a new paradigm in mayfly phylogeny (Ephemeroptera): Combined analysis of morphological and molecular data (2009) Systematic Entomology, 34, pp. 616-634Phillips, D.M., Exceptions to the prevailing pattern of tubules (9+9+2) in the sperm flagella of certain insect species (1969) The Journal of Cell Biology, 40, pp. 28-43Phillips, D.M., Insect sperm: Their structure and morphogenesis (1970) The Journal of Cell Biology, 44, pp. 243-277Regier, J.C., Shultz, J.W., Zwick, A., Hussey, A., Ball, B., Wetzer, R., Martin, J.W., Cunningham, C.W., Arthropod relationships revealed by phylogenomic analysis of nuclear protein-coding sequences (2010) Nature, 463, pp. 1079-1084Rosati, F., Selmi, G., Mazzini, M., Comparative observations on the mitochondrial derivatives of insect sperm (1976) Journal of Submicroscopic Cytology, 8, pp. 51-67Zama, U., Lino-Neto, J., Brito, P., Campos, L.A.O., Dolder, H., Báo, S.N., The sperm morphology of mud dauber Sceliphron fistularium Dahlbom (Hymenoptera: Apoidea: Sphecidae), as an indicative of bees relation (2005) Journal of Submicroscopic Cytology and Pathology, 37, pp. 91-99Zama, U., Lino-Neto, J., Dolder, H., Structure and ultrastructure of spermatozoa in Melliponini (stingless bees) (Hymenoptera: Apidae) (2004) Tissue and Cell, 36, pp. 29-41Zhang, J., Zhou, C., Gai, Y., Song, D., Zhou, K., The complete mitochondrial genome of Parafronurus youi (Insecta: Ephemeroptera) and phylogenetic position of the Ephemeroptera (2008) Gene, 424, pp. 18-24