A detailed rubric initially designed as a scoring instrument for grading APA-style empirical research reports was tested for its ability to help students improve their scientific writing skills. Students who used the rubric while preparing their reports wrote a higher quality report than did students who did not. Students also improved the quality of their own reports after using the rubric to grade a report written by one of their classmates. These findings provide some of the first empirical evidence of the important role that rubrics can play in the process of formative assessment.