In France, PISA science 2006 globally indicated a score about the average, and a particularly high proportion (compared to OECD average) of students at or below level 1, meaning that students are not able to use scientific knowledge to understand and do PISA's easiest tasks. The aims of this project are to investigate links between students' performances in PISA 2006 science (i.e. their scores), students' understanding of the context of the unit, proposed in the unit stimulus or items and competencies involved when the students effectively construct the answer to PISA items. For this reason, we focus our research on the representations and strategies students develop while solving PISA items. We collected audio taped and/or videotaped data from 15 year old students from grade 9 in middle secondary school and grade level 10 in upper secondary school in order to analyse stu-dents' oral and written productions when they answer PISA questions which we had previously selected. After a review of the different research studies developed about PISA secondary analysis, we present our results show-ing that representations and strategies that students construct when answering questions differ between high and low achievers. Our data also indicate for the same PISA question, that competencies students use when they construct their answer may differ from competencies that PISA claims to evaluate. Our results show that differ-ent interpretations can be made about the actual meaning of PISA measure of scientific literacy. In particular, for low achievers, competencies should be refined.