The notion of deliberated teacher reflection as a means of improving professional teaching practice has become one of the most pervasive concepts to influence science teacher education during the past decade. In this case study, we use the notion of teacher reflection and Lytle and Cochran-Smith's (1990) typology of teacher research to examine the relationships between a beginning high school chemistry teacher's articulated personal practice theories and his actions as demonstrated by his curricular decisions and instructional practices. In so doing, we describe the ways in which the formal articulations of this teacher's personal practice theories lent themselves to his development as a teacher/researcher and propose this development as a useful model of science teacher practitioner professionalism. © 2001 John Wiley & Sons, Inc. J Res Sci Teach 38: 408–441, 2001