Two common goals of science teacher professional development (PD) are increased content knowledge (CK) and improved readiness to teach through inquiry. However, PD assessment challenges arise when the context is structured around inquiry-based, participant-driven learning, and when the content crosses scientific disciplines. This study extended the use of concept mapping as an assessment tool for examining changes in the content knowledge of 21 high school science teachers who participated in a field-based environmental science summer institute. The scoring rubric focused on documenting concepts, links, and map organization and scope in an attempt to capture development of cross- disciplinary knowledge in ways that correspond with theories of expertise development. The analysis revealed significant gains from pre-PD to post PD maps in the sophistication of links between concepts and in the number of additional, participant-generated scientifically valid concepts. Relative to the initial maps, post PD maps also manifested more complete clustering of concepts. Findings are discussed in reference to previous studies on teachers’ learning and implications for future research using concept mapping as a means of assessing teacher PD.