ABSTRACTThis case study explores the effects of instruction on the acquisition of second language pronunciation. The study examines whether tutoring sessions using Han's (2001) fine-tuning instruction led to changes in the adult learner's persistent errors in vowel pronunciation over a period of 10 weeks and whether these changes are retained in a delayed posttest. Specifically, the research examined Korean vowel pronunciation acquired by a Japanese learner. Data from ten 2-hour tutoring sessions between a native Korean speaker and the Japanese learner were video recorded and subsequently transcribed for analysis. Twenty-three native speakers of Korean used a questionnaire to rate the learner's pronunciation at three time-points; the learner also self-evaluated her pronunciation through reflective journal entries. To map the changes in vowel production, an acoustic analysis was performed on the vowel production at three time points. Results suggest that fine-tuning instruction is effective in improving persistent pronunciation errors and in retaining this improvement.