The author reports that science and non-science elective teachers, representing different level of subject expertise, were studied. Teachers were interviewed on how they planned a primary science lesson. Interview protocols were analyzed by using the Taxonomy of Observed Teacher Thinking which assessed the quality of teacher thinking as reflected by cognitive complexity. The planned lessons were observed and then evaluated with the Configuration Checklist. The author notes that marked differences were observed in teacher thinking during lesson planning, in structural complexity of teacher thinking, and in teachers' pedagogical content beliefs.