Research on the relation between students’ achievement (ACH) and their academic self-concept (ASC) has consistently shown a Big-Fish-Little-Pond-Effect (BFLPE); ASC is positively affected by individual ACH, but negatively affected by school-average ACH. Surprisingly, however, there are few good UK studies of the BFLPE and few anywhere in the world based on science self-concept (S-ASC). Addressing this substantive limitation in existing research with data from PISA 2006, we extend new multigroup doubly-latent multilevel structural equation models – a substantive-methodological synergy. BFLPE predictions for S-ASC are supported for: the total international sample; the total UK sample; each of the four UK countries considered separately. The BFLPE was marginally larger in the UK than the international sample. However, consistent with the selective nature of school systems in the UK, the BFLPE was larger in Northern Ireland and, to a lesser extent, England than in Scotland or Wales.