The form of a determiner is dependent on different contextual factors: in some languages grammatical number and grammatical gender determine the choice of a determiner variant. In other languages, the phonological onset of the element immediately following the determiner affects selection, too. Previous work has shown that the activation of opposing determiner forms by a noun's grammatical properties leads to slower naming latencies in a picture naming task, as does the activation of opposing forms by the interaction between a noun's gender and the phonological context. The present paper addresses the question of whether phonological context alone is sufficient to evoke competition between determiner forms. Participants produced English phrases in which a noun phrase's phonology required a determiner that was the same as or differed from the determiner required by the noun itself (e.g., "a purple giraffe"; "an orange giraffe"). Naming latencies were slower when the phrase-initial determiner differed from the determiner required by the noun in isolation than when the phrase-initial determiner matched the isolated-noun determiner. This was true both for definite and indefinite determiners. The data show that during the production of a determiner-noun phrase, nouns automatically activate the phonological forms of their determiners, which can compete with the phonological forms that are generated by an assimilation rule. (Contains 2 figures and 2 tables.)