Abstract: By a theorem of Thurston, in the subgroup of the mapping class group generated by Dehn twists around two curves which fill, every element not conjugate to a power of one of the twists is pseudo-Anosov. We prove an analogue of this theorem for the outer automorphism group of a rank n free group. As an application, we show that every element of GL(n,Z) is induced by a nongeometric fully irreducible automorphism. This is joint work with Matt Clay.