The purpose of this cluster-randomized control field trial was to examine whether kindergarten teachers could learn to differentiate classroom reading instruction using Individualized Student Instruction for Kindergarten (ISI-K) and to test the efficacy of differentiation on reading outcomes. The study involved 14 schools, 23 ISI-K (n = 305 students) and 21 contrast teachers (n = 251 students). Data sources included classroom observations, parent surveys, and student assessments of language, cognitive, and reading skills. Hierarchical multivariate linear modeling revealed that students in ISI-K classrooms outperformed contrast students on a latent measure of reading skills (ES = 0.52). Teachers in both conditions provided small-group instruction, but teachers in the ISI-K condition provided significantly more individualized instruction. Findings are discussed regarding professional development to differentiate core reading instruction and the challenge of using Response to Intervention approaches to address students' needs in the areas of reading. (Contains 4 figures and 7 tables.)