Sexual orientation has been conceptualized by Ross and Paul (1992) as fixed in terms of sex object (exclusively heterosexual or homosexual) versus fluid (bisexuals, for whom sex of partner may be unimportant). The authors of the present article investigated characteristics of 1,913 young men and women with fixed and fluid sexual orientation recruited in an Internet-based study of sexual behavior in Sweden. One half of the respondents were students at a major Swedish university. Data indicated that women were twice as likely as men to report a fluid orientation. There were major differences between men and women, with maximal fluidity in the 25 to 34 age range, fluid women being more likely to be urban and not religious, and fluid men reporting higher education levels. Higher sexual partner numbers were associated with fluidity in women but fixedness in men. For both, living away from the family was associated with fluidity. These data suggest that fluidity is more prevalent in women, and that it may be associated not with an unformed sexual preference but with an expanding one in the midtwenties. Conceptualizing sexual orientation as fixed or fluid may be a useful alternative approach to conceptualization of sexual orientation as homosexual, heterosexual, or bisexual.