Reported are the results of a multiyear study in which reading comprehension and writing were integrated within an in-depth science instructional model (Science IDEAS) in daily 1.5 to 2 h daily lessons on a schoolwide basis in grades 3-4-5. Multilevel (HLM7) achievement findings showed the experimental intervention resulted in significant and consistent direct effects in grades 3-4-5 and complementary transfer effects in grades 6-7 on both ITBS Science (+1.08 Grade Equivalent Units [GE]) and ITBS Reading (+0.57 GE). Discussed are implications of the findings and related research for changing grade K-5 curriculum policy to allocate increased instructional time for integrated science instruction.