The genus Axonopits, created by Palisot de Boauvois in 1812, is nearly confined to the tropical and subtropical regions of America, its species, being particularly numerous in Brazil. The taxonomic status of the genus has been a matter of continuous controversy, some authors refusing to accept its validity and prefering to consider it as a simple section of the genus Paspalwm. Nevertheless, the majority of modern agrostologists do recognize Axonopu.s as a real genus, adopting the generic concept, proposed by Chase in 1811, who established once for all the foundations of the genus. In so doing, Chase conside?ed as capital generic characteristics for Axonopus the reversed and solitary spikelets (in which the first glume is wanting), and the racemes aggregated at the summit of the culm. Adopting this same concept as to the validity of the genus, the author presents in this paper a detailed morphological study of 38 Brazilian species, distributed in the same three sections established previously by Chase, namely: 1) Section Enaxonopus (28 species), 2) Section Cabrera (8 species), and 3) Section Lappagopsis (2 especies). Brief references are also made of six other species reported for Brazil, hut which on account of the author's lack of botanical material and general informations, could not he studied in details. Besides giving a morphological account of the species and their distribution in Brazil, the author provides also keys for the classification, of the sections and species; finally he studies the enormous synonymy that has been developed around the genus Axonopus, listing the valid binomies and their synonyms in a final chapter. Four new combinations are herein proposed, representing species formely included in the genus Paspalum. These are: Axonopus complanatus (Neos) nov. comb., A. polydactylus (Steud.) nov. comb., A. ulei (Häck.) nov. comb, and A. triglochinoides (Mez) nov. comb, which are based upon Paspalum complanatum Nees, P. polydactylon Steudel, P. ulei Häck., and P. triglochinoides Mez, respectively.