This study uses implementation fidelity data from PreK to 1st grade in the Boston Public Schools (BPS) to identify profiles of instructional alignment and examine whether stronger alignment for PreK attenders is associated with sustained benefits of BPS PreK on children's language, literacy, and math skills through first grade. The study includes N = 498 students (mean age = 5.47, SD = 0.30 in K fall). Children who experienced strong instructional alignment across grades had faster gains in literacy (SD = .47) and math (SD = .28) skills through the spring of first grade compared with non-BPS PreK attenders. Mis-alignment predicted faster convergence in literacy skills. Results highlight that both strong fidelity in PreK and better instructional alignment through elementary school may help to sustain the initial benefits of PreK programs through first grade in a subset of outcome domains. Implications for research measuring alignment in a broader range of settings and implications for practice are discussed. (author abstract)