In this study, the author developed a model to describe academic self-concept (ASC) in science and validated an instrument for its measurement. Unlike previous models of science ASC, which envisage science as a homogenous single global construct, this model took a multidimensional view by conceiving science self-concept as possessing distinctive facets including conceptual and procedural elements. In the first part of the study, data were collected from 1,483 students attending eight secondary schools in England, through the use of a newly devised Secondary Self-Concept Science Instrument, and structural equation modeling was employed to test and validate a model. In the second part of the study, the data were analysed within the new self-concept framework to examine learners' ASC profiles across the domains of science, with particular attention paid to age- and gender-related differences. The study found that the proposed science self-concept model exhibited robust measures of fit and construct validity, which were shown to be invariant across gender and age subgroups. The self-concept profiles were heterogeneous in nature with the component relating to self-concept in physics, being surprisingly positive in comparison to other aspects of science. This outcome is in stark contrast to data reported elsewhere and raises important issues about the nature of young learners' self-conceptions about science. The paper concludes with an analysis of the potential utility of the self-concept measurement instrument as a pedagogical device for science educators and learners of science.