Although first-year students in Supplemental Instruction (SI) earned similar average numerical-grades in an introductory biology course as non-SI students, their grade distributions were different: SI students earned fewer D's and F's than non-SI students. SI students who earned A's and B's had similar admissions scores as those who earned D's and F's, but were distinguished by their academic behaviors: they submitted more extra-credit work and came to more classes, help sessions, and office hours than non-SI students. These data indicate that SI can help at-risk students in an introductory biology course to engage in positive academic behaviors and to improve their academic performance. (Contains 2 tables.)