The ability to speak in a second language (L2) requires a certain level of linguistic proficiency, but the ability to "live" in a second language requires a certain level of "sociolinguistic" proficiency. L2 sociolinguistic variables present acquisitional challenges for language learners, since informal discourse features are largely absent from classroom-based input but frequent in native speakers' informal communication. In this dissertation, I examine how L2 sociolinguistic performance can be influenced by a specific social characteristic: the interlocutor's native language status vis-a-vis the language of communication. That is, how does learner speech change in conversation with a native speaker compared to conversation with another learner who shares the same L1? While previous studies have examined this interlocutor characteristic on measures of grammatical proficiency in classroom-based learners, few studies have measured its effect on sociolinguistic performance, especially in highly advanced learners. My data focus on two sociolinguistic features that frequently appear in informal French: "ne"-deletion (ND) and subject doubling (SD).I examine the interlocutor effect on these variables in two groups of learners: study-abroad students at low-advanced proficiency and highly proficient near-native speakers. Both groups were recorded in informal one-on-one conversations with a native and non-native French interlocutor. Study-abroad students demonstrated significantly higher rates of ND and SD (characteristic of more informal, nativelike speech) in conversation with a native French speaker than when speaking with another study-abroad student. Furthermore, a variationist analysis revealed interlocutor language status as the most significant social factor influencing variation for ND and SD. In near-native speakers, only marginal differences in ND and SD frequency were detected across interlocutor language statuses, suggesting a diminishing influence as proficiency increases. The results demonstrate that researchers must be aware of this interlocutor effect when designing tasks that evaluate sociolinguistic performance in learners. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]