Brito, P., Mancini,. K., Salles, F.F., Rizzi, E.A. and Dolder, H. 2015. The sperm of Hexagenia (Pseudeatonica) albivitta Walker (Ephemeroptera: Fossoriae: Ephemeridae). -Acta Zoologica (Stockholm) 96: 160-166.This study describes the sperm morphology of the mayfly Hexagenia (Pseudeatonica) albivitta (Ephemeroptera). Its spermatozoon measures approximately 30 lm of which 9 lm 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 lm, 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.