There is widespread international concern about post-16 participation rates in science, with women's under-representation constituting a particular issue. This paper contributes to these debates through a novel, critical examination of the role of masculinity within boys' negotiations of science aspirations. Drawing on a UK longitudinal study of children's science and career aspirations from age 10 to 14 (including a survey of over 9,000 (Year 6, age 10/11) and 5,600 (Year 8, age 12/13) pupils in England and repeat individual interviews with 92 children (at age 10/11) and 85 (age 12/13), the paper focuses in-depth on repeat interviews with 37 boys. We identify five discursive performances of masculinity, which are related to the boys' (science) aspirations: two are associated with science/related aspirations (termed "young professors" and "cool/footballer scientists") and three characterize boys who aspire otherwise ("behaving/achieving" boys; "popular masculinity" boys and "laddish" boys). Classed patterns across these five discourses are then explored through two cross-cutting phenomena, (1) popular constructions of science as "brainy"/"smart" and (2) the uneven social distribution of "science capital," explaining how each of these are implicated facilitating middle-class boys' identifications from/with science and dissuading working-class boys' aspirations. We argue that these analyses illuminate an "orthodoxy" of science careers which maps closely on to current patterns of participation in post-16 science and which impacts powerfully on who can/not conceive of a career in science as being "for me."