Pedagogical Content Knowledge (PCK) in science is the specific professional knowledge of a teacher, which characterizes and differentiates this type of professional from other science-based professionals. Recent research at an international level shows certain diversity in its definition by experts and also in their suggested composition. This diversity has generated a multiplicity of instruments for assessing PCK. It seems convenient to explore whether or not this variety of proposals hides a convergent underlying and steady structure. If this conceptual structure existed, it could be revealed in empirical studies, confirming or rejecting experts' criteria. As the construct of PCK is so complex, simplified situations should be considered first. Therefore, the present study analyses the possible underlying structure of PCK in elementary science shown by primary school teachers prior to acquiring professional expertise. Two independent studies were conducted in different years with pre-service primary school teachers. PCK in elementary science was assessed with CoRe, an instrument used in many international studies. Results show a steady bi-factorial structure grouping 'instructional analysis' (factor 1) and 'instructional action' (factor 2) components. Between-component relationships were conceptually analogous to those observed in previous empirical studies.