Aegyptos was the son of Belus and Achiroe, a naiad daughter of Nile. He ruled Arabia and conquered nearby country ruled by people called Melampodes and called it by his name. Aegyptus fathered fifty sons, who were all but one murdered by forty nine of the fifty daughters of Aegyptus' twin brother, Danaus, eponym of the Danaids.
A scholium on a line in Euripides, Hecuba 886, reverses these origins, placing the twin brothers at first in Argolis, whence Aegyptus was expelled and fled to the land that was named after him. In the more common version, Aegyptus commanded that his fifty sons marry the fifty Danaides, and Danaus with his daughters fled to Argos, ruled by Pelasgus or by Gelanor, whom Danaus replaced. When Aegyptus and his sons arrived to take the Danaides, Danaus relinquished them, to spare the Argives the pain of a battle; however, he instructed his daughters to kill their husbands on their wedding night. Forty-nine followed through, but one, Hypermnestra ("greatly wooed"), refused, because her husband, Lynceus the "lynx-man", honored her wish to remain a virgin. Danaus was angry with his disobedient daughter and threw her to the Argive courts. Aphrodite intervened and saved her. Lynceus and Hypermnestra founded the lineage of Argive kings, a Danaid Dynasty.
In some versions, Lynceus later slew Danaus as revenge for the death of his brothers, and the Danaides were punished in the underworld by being forced to carry water through a jug with holes, or a sieve, so that the water always leaked out.