This study examined the effects and fidelity of collaborative strategic reading (CSR) implemented by experienced CSR teachers (participated in previous study; Vaughn et al., 2011) on the reading comprehension outcomes of students in English/Language Arts (ELA) or Reading classes. Eligible teachers (12 of 17; others reassigned to teach grades/subjects not eligible for inclusion) in middle schools in Texas and Colorado who participated in the previous year in a study examining the effects of CSR on the reading comprehension outcomes of their students participated in a 2nd-year, new cohort of students. Teachers taught multiple sections of ELA or reading; sections were randomly assigned to a treatment or comparison condition, and any extra classes were assigned to the treatment condition. There were 26 CSR and 22 comparison classes. Teachers were asked to implement CSR in their treatment classes only for approximately two 50-min sessions per week for 18 weeks. Examining the role of fidelity revealed that CSR was more prevalent in treatment classes than the comparison classes and that ELA teachers had significantly less treatment spillover than the Reading teachers. Findings indicate that CSR was associated with a greater effect when implemented in ELA classrooms compared to Reading classrooms. (Contains 6 tables and 1 figure.)