This article examines why numbers have declined in French science universities. It shows that the main causes of this decline are the growth in student numbers between 1985 and 1995 and the changes that this brought about in the educational and social composition of the student population. Since the mid-1990s, increasingly fewer French students have been enrolling for science subjects at university. Speaking of a "loss of interest in science among young people" is nevertheless premature. The phenomenon is primarily due to socio-demographic changes. As a result of their position in French higher education, universities, which do not operate a selective admission policy, are a last resort for students rejected by the selective options. Between 1985 and 1995, when student demographics grew apace, universities absorbed most of this growth. After 1995, the supply of selective training increased, while student demographics stagnated. As a result, university enrolments have declined, especially in the sciences. At the same time, holders of science baccalaureates have not been as top-notch, educationally and socially, as they were fifteen years previously. These students from more modest backgrounds and performing less well in education are shying away from the sciences which are reputed to be more difficult and less "profitable" than other subjects.