female-female pairs of ring-billed gulls were studied on granite island, northern lake superior, during the breeding seasons of 1979 and 1980. in 1979 the colony consisted of approximately 2400 pairs with 99 nests (4.1%) containing five to seven eggs. in 1980 the colony had increased in size to 2600 pairs and contained 71 (2.7%) superclutches. extrapolating from superclutches in my observation areas, where all attendants were known, approximately 85% of the total ntmber of superclutches were attended by female-female pairs. the difficulty distinguishing these nests from those of polygynous groups laying in a single nest-cup or nests receiving dump eggs is discussed. because of these difficulties, all superclutches were monitored and treated as a single sample. eggs laid in superclutches were slightly smaller than those from normal clutches, but did not differ in shape. the incubation period differed significantly for eggs of the two clutch types, with eggs in superclutches taking ionger to hatch than those in normal clutches. ihis may be because the eggs toward the outside edge of nests containing superclutches received significantly less heat. nests containing superclutches were larger than those containing normal clutches but were of similar quality and were not differentially located by substrate, density or colony location. nearest neighbour distance was also similar for the two clutch types. nest success did not differ significantly between superclutches and normal clutches. hatching success for superclutches was 33% in 1979 and 29% in 1980, while normal clutches had 77% and 61% in 1979 and 1980, respectively. significantly more eggs rolled from the nest or were destroyed or abandoned in superclutches compared to those from normal clutches. egg fertility rates were 87% and 94% for eggs in superclutches while normal clutches had 100% and 99% fertility in the two years of study. chicks from normal clutches had a significantly higher fledging success than those of superclutches. chicks from the latter had a higher rate of mortality due mainly to a higher death rate during the week following hatching. during both years of my study chicks from superclutches hatched at significantly lighter weights than those from normal clutches, but their weights did not differ after the first week post hatch. tarsal and culmen measurements followed a similar pattern to that of weight. food types brought to chicks from the two clutch types did not differ. their diet consisted almost totally of fish and insects. females of female-female pairs were of normal size, weight, age and condition compared to heterosexually paired females. levels of 13 blood chemistry parameters were measured for female-female pair members and heterosexually paired birds. the only consistent differences were low cholesterol and elevated progesterone levels in members of female-female pairs. intra-pair size differences suggest that the females assertively paired by size. they exhibited mate fidelity and to a lesser extent nest-site tenacity. behaviour of female-female pairs is compared to heterosexual pairs during courtship, the incubation period and rearing of the brood. females involved in female-female pairs cooperated well. four polygynous groups were monitored, and their behaviour and reproductive success is reported. existing hypotheses regarding the origin(s) of female-female pairing and their biological significance are discussed.