The advent of national standards, formal assessments, and greater state and local oversight requires physical educators to teach predetermined concepts within student-centered formats. This is a dramatic change from freedoms teachers experienced previously, often presenting teacher-directed multi-activities consistent with personal interests and timelines. In this research, we examined adjustments teachers made to a structured curriculum and their rationales for these modifications. This study was part of curriculum-intervention research involving ~ 6700 3rd - 5th grade students in 30 schools. In this design, schools were randomly selected and assigned to experimental and control groups. Physical educators in experimental and control schools received 30 hours of training in either the new (E) or the current (C) curriculum. Following training the15 experimental teachers taught 10 structured lessons to each grade. The lessons emphasized concepts consistent with national standards in science and physical education. Lessons used a consistent format, reflecting the 5 Es of science inquiry (engagement, exploration, explanation, elaboration, evaluation) in cooperative problem solving formats consistent with constructivist approaches. In findings reported elsewhere, concept knowledge increased by 21% in experimental schools vs. 2% in controls. In this study, we used observation and teacher interviews to examine adjustments experimental group teachers made to the structured curriculum. Each teacher was observed weekly for two, 30 min. classes over 6 weeks and all grades and lessons were represented. In hour interviews, teachers explained their perceptions of the new curriculum and changes made during implementation. Data were analyzed using constant comparison and themes were triangulated across school contexts. Results suggested teachers felt uncomfortable initially with structures imposed in their classes. They were surprised by the preparation required to teach lessons they, themselves, had not created. Six (40%) reported they did not want to spend time preparing for teaching or energy to manage students effectively for cognitive engagement. Data suggested these 6 teachers used 2 modifications when they judged their students were not ready for “content-oriented” lessons. First, they “slipped in” additional lessons to introduce concepts before teaching the required lesson. Second, they extended lessons beyond the 30 min. period, at times using 2 periods to teach the prescribed 30 min. lesson. These teachers lacked confidence in the curriculum to increase learning and confidence in their students’ ability to learn concepts in a structured program. Some teachers may need evidence of increased student learning before they are willing to expend the effort to faithfully implement a structured curriculum.Keyword(s): curriculum development, elementary education, interdisciplinary