Although many sociological studies have included attention to the field of sf, little work has been done to develop a specific sociology of sf; that is, to focus on the social dynamics within the world of sf writing and reading, and especially in the peculiar environment of fandom. In the light of studies such as Camille Bacon-Smith's Science Fiction Culture and John Tulloch and Henry Jenkins's Science Fiction Audiences, and in the context of Pierre Bourdieu's theoretical work on the notion of the literary field, this paper surveys the problem of the relationship between sf and mainstream literature, with special attention to the positioning of fandom. The case of the sf field in Spain, which is currently enjoying a period of expansion and a publishing boom that makes this problem increasingly relevant, is analyzed through interviews with writers, editors, and fans. I argue that the various definitions of the genre espoused by different actors in the field function as differing responses from within sf to the problem of exclusion from the mainstream.