Studies were made in 1970 in the Chukotski Peninsula, in 1971 in the delta of the Indigirka river and in 1972 in the delta of the Yana river. Grey Phalaropes inhabit polygonal and tussocky moss-sedge tundra rich in swamps, lakes and (in June) temporary ponds. Population density in favourable habitats may reach 1–2 pairs ha-1. Data on breeding chronology are presented, and various aggressive and courtship displays described. Most phalaropes seem to keep within a home range, sometimes large, during courtship time, but no defended territories and no forms of territorial behaviour exist. Many birds, both local nesters and wanderers, can feed on any pond. Sexual dimorphism is described. In 1970, three non-breeding cock-plumaged females were taken. Pairs are formed both before arrival and on the nesting grounds. All courtship displays are wholly or mostly initiated by females. In 1971, in the Indigirka delta, all the Grey Phalaropes were paired by 12 June, and stayed in pairs until the end of egg-laying. In 1970, on the northern Chukotsk, phalaropes seemed to form no (or very few) permanent pairs. Throughout June, most birds occurred in mixed flocks constantly moving between lakes or ponds. Copulation seemed to be promiscuous within the local population; polyandry cannot be excluded in some cases. Pairs appeared to be created only for the time of egglaying; probably, the only biological role of pair-formation is to find a male for incubating. Thus, a definite social system is not a species-specific feature; it can vary depending on local and yearly situations, including probably sex ratio. Nests are usually situated in very wet places, sometimes on the water edge. They can be found as little as 3 m apart, but are usually 40–80 m apart, or further. Incubation begins after the second or third egg. After the end of egg-laying, males drive females away from the nests, and pairs break up. Females and non-breeders gather in flocks and move onto the lakes of maritime tundra, and later on to the sea. The composition of the flocks is not constant: they often join together or part. Brooding males feed near their nests, sometimes in groups; not unfrequently they join flocks of females and non-breeders for some time. The normal average clutch-size is c. 4 eggs; when nesting was delayed (in the central Indigirka delta in 1971) the average was 3–61. The loss of nests was great in 1971; numbers of young on 1–3 August was 10 times lower than adult numbers in June.