Courses that interfere with undergraduate students' persistence are barriers that appear all along the undergraduate continuum. Supplemental Instruction (SI) may contribute to students' achievement in a barrier course and, therefore, to their persistence in their academic program. The purpose of this single-case descriptive study was to explore student and instructor perceptions of SI in an upper-level chemistry course with a reputation for being a barrier to academic success. The case study methodology used included a focus group, one-on-one interviews with instructors and students, document review, and class and SI statistics. Results indicated that faculty and students perceived SI to be a valuable resource in achieving persistence or academic success. (Contains 4 figures.)