Performance on intelligence tests is known to be associated with class mobility, with high scorers tending to move up the socio-economic hierarchy, and low scorers tending to move down. However, much remains unknown about the association. It is possible that the importance of intelligence varies across different occupational areas, or that there is friction acting against mobility, such that a person from an underprivileged background would have to be more intelligent in order to reach a given position than someone who had had greater social advantage. Data from a longitudinal study of a broad, socially representative cohort of the British population (the NCDS) are used to investigate these questions. The results show that intelligence test scores in childhood are associated with class mobility in adulthood uniformly across all social classes. There is no evidence that those from underprivileged backgrounds have to be disproportionately able in order to reach the professional classes. The study reveals an apparently high level of social mobility and meritocracy in contemporary Britain.