This study investigates the effects of the Science Writing Heuristic (SWH), known as an argumentation-based science inquiry approach, on Grade 9 students' performance on a post-test in relation to their academic achievement levels. Four intact classes taught by 2 chemistry teachers from a Turkish public high school were selected for the study; one class from each teacher was assigned as the treatment group, and the other class from each teacher was assigned as the control group. Students in the treatment group were instructed using the SWH approach while those in the control group were instructed using traditional instruction. Students' achievement levels were determined based on their chemistry mean scores for the previous semester, which were used to determine the impact of the treatment on varying levels of achievement. A test measuring students' achievement in chemical change and mixture was administered as pre- and post-test for both groups. The data were analyzed by using Analysis of Covariance (ANCOVA). The findings indicate that the SWH approach contributed to students' test performances significantly better than the traditional approach. Student performance on the post-test differed with respect to their academic achievement levels significantly. Low achievers and middle achievers in the SWH group significantly outperformed those in the traditional group on the post-test. (Contains 2 tables and 2 figures.)