Most theories of implicature make reference to a notion of alternatives.Interlocutors reason about what the speaker could have said. In this paper, I investigate the structure of these alternatives. In particular, I ask how these alternative utterances are constrained by the interlocutors' grammar. I argue that in order to derive certain implicatures, alternative utterances must be analyzed like actual utterances, as fully compositional structures appropriately generated by the grammar. The data supporting this position come from implicatures generated by indefinite bare noun phrases in Tagalog. I show that Tagalog indefinites give rise to nonuniqueness implicatures via competition with definites, as in English. However, unlike English, definite and indefinite interpretations of Tagalog NPs are not signalled by dedicated articles, but by verbal affixes. Therefore, in order to generate the observed implicatures, pragmatic competition must take into consideration the NP's broader syntactic context. Supporting the view that implicature calculation is sensitive to the morphosyntactic structure of alternative utterances, I show that in cases where the alternative is not grammatically well-formed, the implicature does not arise. These data provide evidence that only grammatically well-formed structures are able to enter into pragmatic competition.