The study examined how learners' motivational dispositions predict the complexity, accuracy, fluency of their oral task performance and their overall second language proficiency. Eighty-one speakers of English as a second language in the United States completed a regulatory focus and a regulatory mode questionnaire, an oral interview task, and a picture description task. Regression analyses showed a meaningful pattern of results. The assessment mode (concerned with analysis, evaluation, and comparison) contributed to syntactic and lexical complexity, the reduction of dysfluencies, and overall English proficiency, whereas the locomotion mode (concerned with acting without deliberation) positively predicted the speed measure of fluency and the total number of errors. The prevention focus (concerned with stability, security, and safety) negatively predicted lexical sophistication, whereas the promotion focus (concerned with advancement, accomplishments, and growth) predicted overall English proficiency. Theoretical and instructional implications were discussed.