A first attempt to homologize male palpal bulb structures of theraphosine spiders is made, with the aim of providing systematic characters. The morphology and distribution of palpal bulb keels of over 60 species in 27 genera of Theraphosinae is presented and discussed. Four basic groups of keels were recognized and a terminology was created to name them: prolateral inferior and prolateral superior keels, for the two more or less parallel keels found on the prolateral bulb face; apical keel, for the ventral keel located just before the apex of the embolus; subapical keel, for a keel located just before the apical keel; and, retrolateral keel, for the keel located on the retrolateral region, originating on the apical region and extending backwards. Other palpal bulb keels, apart from these four basic groups, as well as other structures, were found in some genera and/or species, constituting apomorphies for these groups.