Many state and federal governments have mandated in such documents as the National Science Education Standards that inquiry strategies should be the focus of the teaching of science within school classrooms. The difficult part for success is changing teacher practices from perceived traditional ways of teaching to more inquiry-based approaches. Arguments are often made about the effectiveness of these traditional strategies. The purpose of this study was to compare the effectiveness of the inquiry-based approach known as the Science Writing Heuristic approach as a treatment to traditional teaching practices on students' post-test scores in relation to students' achievement level and teacher's implementation of the approach. A mixed-method research approach was used to analyze the teacher observational data and students' test results. The major findings of this study are that the quality of the implementation does have an impact on student performance on post-test scores and that high-quality implementation of the Science Writing Heuristic approach has significant advantages in closing the achievement gap within science classrooms. (Contains 6 tables and 4 figures.)