Lucretius’ De rerum natura is one of the relatively few corpora of Greek and Roman literature that is structured in six books. It is distinguished as well by features that encourage readers to understand it both as a sequence of two groups of three books (1+2+3, 4+5+6) and also as three successive pairs of books (1+2, 3+4, 5+6). This paper argues that the former organizations scheme derives from the structure of Ennius’ Annales and the latter from Callimachus’ book of Hynms. It further argues that this Lucretius’ union of these two six-element schemes influenced the structure employed by Ovid in the Fasti. An appendix endorses Zetzel’s idea that the six-book structure of Cicero’s De re publica marks that work as well as a response to Lucretius’ poem.