Teaching science through inquiry-based, student-centered instructional methods has been consistently emphasized by science education reform documents such as the National Research Council's (NRC) "National Science Education Standards" (NSES), and practically all states have adopted inquiry standards. In order for science education reforms to succeed, it is necessary for teachers to be familiar with and utilize inquiry-based practices in their classrooms; however, this is not the case in many classrooms around the country. Although, there may be numerous explanations to account for this unfortunate phenomenon, one of the most important reasons to recognize and address is teachers' lack of familiarity with and inability to effectively employ inquiry-based instructional methods in their classrooms. Professional development as a tool to enhance teaching is especially stressed in science education reform documents (e.g. NSES) that emphasize inquiry teaching; however, as suggested by prior studies, not all professional development experiences can be defined as successful and fruitful. Professional development programs should model inquiry-based instruction and allow teachers opportunities to experience science inquiry in an active, collaborative setting and through authentic inquiry research. Beginning in 2003, one such professional development program has allowed high school science teachers in a particular Midwestern state to have opportunities to experience science inquiry first-hand and learn about inquiry-based teaching. An earlier case-study by Lotter, et al. (2007) involved three high school science teachers who participated in a two-week inquiry-based professional development workshop and reported on the type and degree of change in four core conceptions: (1) conceptions of science; (2) conceptions of students and student learning; (3) conceptions of effective teaching practices; and (4) conceptions about the purpose of education. This current case study focuses on a participant attending the same professional development program two years later whose teaching assignments included three different courses. The study aims to explore the changes in the core conceptions and instructional practices of this teacher with regard to all three courses. Furthermore, factors that aid or inhibit the implementation of inquiry-based teaching in these different courses are examined. (Contains 1 figure.)