The purpose of this study was to examine the ways in which elementary teachers applied their understanding of conceptual learning and teaching to their instructional practices as they became knowledgeable about conceptual change pedagogy. Teachers' various ways to interpret and utilize students' prior ideas were analyzed in both epistemological and ontological dimensions of learning. A total of 14 in-service elementary teachers conducted an 8-week-long inquiry into students' conceptual learning as a professional development course project. Major data sources included the teachers' reports on their students' prior ideas, lesson plans with justifications, student performance artifacts, video-recorded teaching episodes, and final reports on their analyses of student learning. The findings demonstrated three epistemologically distinct ways the teachers interpreted and utilized students' prior ideas. These supported Kinchin's epistemological categories of perspectives on teaching including positivist, misconceptions, and systems views. On the basis of Chi's and Thagard's theories of conceptual change, the teachers' ontological understanding of conceptual learning was differentiated in two ways. Some teachers taught a unit to change the ontological nature of student ideas, whereas the others taught a unit within the same ontological categories of student ideas. The findings about teachers' various ways of utilizing students' prior ideas in their instructional practices suggested a number of topics to be addressed in science teacher education such as methods of utilizing students' cognitive resources, strategies for purposeful use of counter-evidence, and understanding of ontological demands of learning. Future research questions were suggested. (Contains 5 tables and 1 figure.)