Aegyptos was the son of King Belus of Egypt and Achiroe, a naiad daughter of Nile. He was the twin brother of Danaus, king of Libya while Euripides adds two others, Cepheus, king of Ethiopia and Phineus, betrothed of Andromeda.
Aegyptus fathered 50 sons by different women: six of whom by a woman of royal blood called Argyphia; 10 by an Arabian woman; seven by a Phoenician woman; three by Tyria; 12 by the naiad Caliadne; six by Gorgo and lastly another six by Hephaestine. According to Hippostratus, Aegyptus had these progeny by a single woman called Eurryroe, daughter of Nilus. In some accounts, Aegyptus consorted with Isaie while Danaus married Melia, these two women were daughters of their uncle Agenor, king of Tyre, and of their possible sister, Damno who was described as the daughter of Belus.
Aegyptus ruled Arabia and conquered nearby country ruled by people called Melampodes and called it by his name, Egypt. 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.
Genealogy of Aegyptus
- Egypt took its name from his, according to folk etymology (see the article Copt); thus for Euripides, in his tragedy Helen, Aegyptus has become Egypt itself: "Proteus, while he lived, was King here, ruling the whole of Aigyptos from his palace on the island of Pharos."
- "Belos", "lord", is simply a Hellenized rendition of Baal, a Semitic term, not an Egyptian one.
- Pseudo-Apollodorus. Bibliotheca, 2.1.5
- John Tzetzes. Chiliades, 7.37 p. 368-369 Translated by Vasiliki Dogani
- Scholia on Apollonius Rhodius, Argonautica, Notes on Book 3,1689
- Pseudo-Apollodorus. Bibliotheca, 2.1.4-5
- An eponym for autochthonous peoples, here represented as pre-Hellenic.
- Stewart, M. People, Places & Things: Aegyptus (1), Greek Mythology: From the Iliad to the Fall of the Last Tyrant. 
- Jean Vertemont, Dictionnaire des mythologies indo-europeenes, 1997.