Sesostris I, d. 1926 B.C., king of ancient Egypt, 2d ruler of the XII dynasty; son and successor of Amenemhet I. He was coregent with his father from 1980 B.C.; from 1971 to 1926 he was sole ruler. His reign was notable for successful campaigns in Nubia. Sesostris' son and successor, Amenemhet II (see under Amenemhet I), was his coregent during the last three years of his reign. Sesostris II, d. 1878 B.C., was the son of Amenemhet II, first his coregent (1900 B.C.), then his successor (1897-1878). Sesostris III, d. 1840 B.C., succeeded (1878 B.C.) his father Sesostris II. He fixed the southern boundary of Egypt above the Second Cataract of the Nile not far from Wadi Halfa and curbed the power of the Egyptian nobles.