It has been assumed that combining living in a native speech community with formal classroom instruction creates an ideal learning environment for foreign language learners. This study examines the extent to which formal instruction affects the oral discourse of advanced learners in target-language speaking environments. From a discourse perspective, the study compares the speech of native speakers of Spanish with participants who received four weeks of grammar instruction on a difficult structure--the Spanish subjunctive--while studying in a graduate program in Mexico for two months. While no statistically significant difference was found in participants' use of the target structure before and after instructional treatment, the study suggests that some participants were able to compose tightly structured argumentation over time and only a learner at the Advanced-High level could produce native-like discourse in hypothetical situations.