The earliest record of young (135 mm in length and 26 grams in weight) was at Kaolak River on July 14, 1951. On July 9, 1952, at Topagaruk the oviduct of an adult female, 86 grams in weight, contained an egg in a shell 200 mm in diameter. Her second largest ovum was 10 mm. Breeding males on this date had testes averaging 11 mm in length. The average length of testis of 15 juveniles shot on August 3, 1951, at Teshekpuk Lake was 1.9 (1.5-2.0) mm. The average weight of these juveniles was 60 (50-81) grams. A comparison of male and female juveniles shows no significant differences. Nevertheless, adult males in both the breeding and post-breeding seasons are longer bodied and heavier than adult females. In the period June 14-25, 1952, in the Point Barrow area, pectoral sandpipers were puffing their throats and cooing. On June 23, several birds were defending territories, and one half mile northeast of Barrow Village (June 23, 1952) we noted a male pectoral sandpiper that crouched low when a pomarine jaeger flew directly overhead. After the jaeger passed, the sandpiper assumed normal posture and continued feeding. At Topagaruk (July 7, 1951) these birds represented less than one per cent of the avian population, were common on polygons having low centers, and frequently joined black-bellied plovers, ruddy turnstones, and semipalmated sandpipers to form discrete flocks. On a four hour field trip at Kaolak River (July 15, 1951), the pectoral sandpipers (45 by actual count) were the most common of the sandpipers and were always calling overhead. The young on this date were not yet capable of flight and were being fed by adult females. One of the immatures bathed in water at the edge of the beach. On July 18, females were still attempting to decoy intruders by pretending to have broken wings. Eight adults with young were observed at Kaolak (June 21-27, 1951) but the species was not so aggressive as at Kaolak River, nor so numerous. The fewer birds may have been correlated with lack of sand dunes, river beaches and open areas.