TY - INPR N2 - This study examined how longitudinal interaction impacts the development of second language (L2) oral proficiency in relation to learners? different experience and proficiency levels. Japanese English-as-a-Foreign-Language learners participated in weekly conversation exchanges with native speakers (NSs) in the US via videoconferencing tools over one academic semester (12 weeks). The participants? spontaneous speech, elicited from a story telling task before and after the treatment, was analyzed via a set of linguistic measures. In line with the componential view of L2 oral proficiency (De Jong et al., 2012) and development (Bundgaard-Nielsen et al., 2011), our results hinted L2 learners? experience and proficiency levels as a mediating factor for determining the link between interaction and its impact on different dimensions of L2 speech learning. While the longitudinal interaction equally improved the participants? grammatical complexity and articulation rate?a fundamental component for defining L2 oral proficiency, the development of less experienced/proficient learners was observed across a wide range of lexicogrammar and fluency features (lexical appropriateness/richness, grammatical accuracy, pause ratio). It was only more experienced/proficient learners that significantly enhanced phonological accuracies (segmentals, word stress) which are thought to gradually develop in the later stages of L2 speech learning. These findings add another piece of evidence for the differential effects of long-term interaction relative to L2 learners? developmental stages. A1 - Saito, K A1 - Suzuki, S A1 - Oyama, T A1 - Akiyama, Y PB - SAGE Publications Y1 - 2019/11/14/ N1 - This version is the author accepted manuscript. For information on re-use, please refer to the publisher?s terms and conditions. SN - 0267-6583 ID - discovery10083749 AV - public JF - Second Language Research UR - https://doi.org/10.1177/0267658319884981 KW - Interaction KW - Feedback KW - Second Language Speech KW - Computer Assisted Language Learning TI - How Does Longitudinal Interaction Promote Second Language Speech Learning? Roles of Learner Experience and Proficiency Levels ER -