This study examines Title 1 heterogeneous classroom teachers' instructional behavior change through implementing well-designed research-based curriculum units and attending regular professional development activities across 3 years. Employing an experimental design, this study compares experimental and comparison teachers' behavioral changes as measured by an observation scale of differentiated teaching strategies across 3 years. The results show that experimental teachers received statistically significant and educationally important higher ratings than comparison teachers on differentiated strategy use and effectiveness across 3 years. The study corroborates the research literature that shows that teachers' instructional improvement takes 2 years to manifest its effectiveness and to shape belief in student learning benefits. (Contains 7 tables.)