This is a narrative inquiry into the role of professional development in the construction of teaching practice by an exemplary urban high school science teacher. I collected data during 3 years of ethnographic participant observation in Marie Gonzalez's classroom. Marie told stories about her experiences in ten years of professional development focused on inquiry science teaching. I use a social practice theory lens to analyze my own stories as well as Marie's. I make the case that science teaching is best understood as mediated by socially-constructed identities rather than as the end-product of knowledge and beliefs. The cognitive paradigm for understanding teachers' professional learning fails to consistently produce transformations of teaching practice. In order to design professional development with science teachers that is generative of new knowledge, and is self-sustaining, we must understand how to build knowledge of how to problematize identities and consciously use social practice theory.