This study sought further evidence of the concurrent validity of concept mapping as a research and evaluation tool in science education. Specifically, the study examined the extent to which differences exist in the concept maps of advanced college biology majors (N = 25) and beginning nonmajors (N = 25) in the domain of mammals. Furthermore, it explored whether these differences are reflected in the way subjects assign class membership as revealed in a card sorting task. The results indicate that concept maps of biology majors are structurally more complex than those of nonmajors and that differences in the structural complexity and organizational patterns depicted in concept maps are reflected in the underlying dimensions used to assign class membership. Together, these findings suggest that the concept map provides a theoretically powerful and psychometrically sound tool for assessing conceptual change in experimental and classroom settings.