In the lesbian community, one which based upon a shared sexual minority identity, recent attempts to add the category "bisexual" to the prevailing dichotomous coceptualization of sexuality have led to various popular conceptualizations of sexuality. Lesbian-identified women disagree among themselves and with bisexual-identified women over whether bisexuality exists, and if so, what it is. As a result, individuals develop lesbian and bisexual identities based on differing conceptions of sexuality, thereby undermining the basis for affiliation among women with a shared sexual identity. This paper, based upon data from 365 lesbian- and bisexual-identified women who were questioned about their sexual identity histories, behaviors, and feelings of sexual attraction, demonstrates that while there are aggregate differences between the experiences of lesbian and bisexual women, there is also a wide range of sexual experience common to both groups. The paper argues that the tension which characterizes relations between lesbian- and bisexual-identified women is not the result of failure to recognize these similarities in experience. Instead, historical circumstances have led to a situation in which bisexuality poses a personal and political threat to lesbians and lesbian politics; the similarity between lesbians' and bisexuals' experiences aggravates rather than mitigates this threat.