Investigations of the colonies of western gulls on Santa Barbara Island, California, have revealed a surplus of females and the occurrence of female-female pairs that produce clutches with as many as six eggs. Females are able to establish and defend breeding territories, behaviours generally thought to be under the control of androgens. There are very few significant differences in circulating levels of the luteinizing hormone and androgens among breeding males, breeding females in heterosexual pairs, and breeding females in homosexual pairs. In contrast, however, only females sampled in 1877, in both homo- and heterosexual pairs, have elevated plasma levels of oestrogens, in spring, coincident with the period in which they show courtship behaviours such as food begging and solicitation of copulation. Given a sex ratio skewed in favour of females, as is apparently the case with the colonies of this species on Santa Barbara Island, and the essentially equal plasma levels of androgens in males and females, it is not difficult to rationalize the formation of female-female pairs. Our findings do not support the hypothesis that female-female pairing involves hormonal masculinization of one member of the pair. © 1982.