This paper presents results from a qualitative study of perceptions of science careers in the Centre National de la Recherche Scientifique (CNRS), the main research institution in France. Its aim is to understand the 'glass ceiling' effect, which reduces the proportion of women at the higher levels of the career hierarchy. Long interviews were carried out with men, as well as women, support staff and researchers. Factors such as tension between individual and collective dimensions of research activity, and long-term time-management problems, were identified: these affect both men and women but in different ways. Organizations bear an important responsibility through the way they reinforce or alleviate difficulties that women and men face in contributing to scientific research at all levels.