Abstract: The ultrastructure of bivalve spermatozoa can be species-specific and often provide valuable taxonomic traits for systematic reviews and phylogenetic reconstructions. Transmission and scanning electron microscopy was an efficient tool to differentiate the spermatozoan structure of species sharing many conchological characters, which could lead to equivocal identification. We analyzed sperm ultrastructural features that were useful for taxonomic issues with regard to Isognomon bicolor and Isognomon alatus (Isognomonidae); Macoma constricta, Macoma biota and Tellina lineata (Tellinidae); Donax hanleyanus and Donax gemmula (Donacidae). The data revealed that the bivalve species studied here, except T. lineata, produce primitive-type spermatozoa, typical of marine invertebrates in which eggs are fertilized in the surrounding water. We, also, compared the ultrastructure of spermatozoa from the bivalves Anomalocardia brasiliana and Tivela mactroides (Veneridae). The spermatozoa of both species contain a curved nucleus with a short cone-shaped acrosome. There were six mitochondria and glycogen clusters in the middle piece of the T. mactroides sperm cell. Possibly, the glycogen clusters and the higher mitochondria number correspond to an adaptive advantage in turbulent waters. Increasing the sperm life expectance also increases the odds of finding the eggs and accomplishing fertilization. Oysters of the genus Crassostrea and mussels of the genus Mytella, commercially important in Brazil, were also investigated in this thesis. The sperm ultrastructure of the specimens described herein and molecular studies, confirm the presence of Crassostrea brasiliana (Ostreidae) in Cananéia. Spermatozoa of mussel specimens of the genus Mytella, from two populations living in distinct habitats (muddy-sand sediment and rocky shores), were examined by transmission electron microscopy. The spermatozoa of all specimens shared the same ultrastructural pattern suggesting that the analyzed specimens, which live in distinct habitats, indeed belong to a same species, which was identified as M. charruana (Mytilidae) based on conchological analysis. The presence of an axial rod in sperm cells upholds the inclusion of M. charruana in the subfamily Mytilinae. In conclusion the electron microscopy was an efficient method to solve some particular taxonomic issues. The present spermatozoan ultrastructural studies have provided new and welcome data about the families Isognomonidae, Veneridae, Tellinidae, Donacidae, Ostreidae and Mytilidae. The results showed in this thesis could be valuable in future phylogenetic analyses. Advisors/Committee Members: UNIVERSIDADE ESTADUAL DE CAMPINAS (CRUESP), Recco-Pimentel, Shirlei Maria, 1954- (advisor), Universidade Estadual de Campinas. Instituto de Biologia (institution), Programa de Pós-Graduação em Biologia Celular e Estrutural (nameofprogram), Kawano, Toshie (committee member), Arruda, Eliane Pintor de (committee member), Matos, Edilson Rodrigues (committee member), Joazeiro, Paulo Pinto (committee member).