Little is known empirically about intraclass correlations (ICCs) for multisite cluster randomized trial (MSCRT) designs, particularly in science education. In this study, ICCs suitable for science achievement studies using a three-level (students in schools in districts) MSCRT design that block on district are estimated and examined. Estimates of ICCs, by district, are computed for unconditional two-level (students in schools) hierarchical linear models using student-level science achievement raw scores as measured by the Texas Assessment of Knowledge and Skills in 2010–2011 for Grades 5, 8, 10, and 11. The average within-district ICCs for MSCRTs of science achievement range from 0.0781 to 0.0982 across grades. For Grade 5, a significant difference is found in the ICC estimate suitable for MSCRTs with many districts versus only a few districts. Consequently, evaluators of science education interventions should exhibit care when selecting ICC estimates for three-level MSCRT designs that block on district.