Purpose: This study examined preschool teachers' fidelity to the language-focused curriculum (LFC; B. Bunce, 1995), a comprehensive classroom curriculum designed to improve at-risk children's language outcomes through targeted improvements to a classroom's activity contexts (e.g., dramatic play, art, storybook reading) and instructional processes (e.g., teacher use of open-ended questions, recasts, and expansions). Specific aims included to (a) examine program differentiation by determining how measures of activity contexts and instructional processes differentiated treatment and comparison teachers, (b) determine treatment teachers' adherence to both activity contexts and instructional processes over an entire academic year, and (c) determine treatment teachers' reported quality of program delivery and comfort with curriculum implementation. Method: Fourteen preschool teachers were randomly assigned to implement the LFC or to maintain their prevailing curriculum. Fidelity was measured 3 times over an academic year using a curriculum fidelity checklist. Results: LFC teachers exhibited fidelity to activity contexts more readily than to instructional processes. Teacher use of language-focused instructional processes was relatively low even after a year of LFC implementation. Conclusion: This study supports the need for speech-language pathologists to work closely with preschool educators to implement the activity contexts and instructional processes associated with high-quality preschool language-learning environments.