A son of Agenor and Telephassa, and brother of Europa, Phoenix, and Cilix. When Europa was carried off by Zeus to Crete, Agenor sent out his sons in search of their sister, enjoining them not to return without her. Telephassa accompanied her sons. All researches being fruitless, Cadmus and Telephassa settled in Thrace. Here Telephassa died, and Cadmus, after burying her, went to Delphi to consult the oracle respecting his sister. The god commanded him to abstain from further seeking, and to follow a cow of a certain kind, and to build a town on the spot where the cow should sink down with fatigue. (Schol. ad Eurip. Phoen. 638, ad Aristoph. Ran. 1256; Paus. ix. 12. § 1.) Cadmus found the cow described by the oracle in Phocis among the herds of Pelagon, and followed her into Boeotia, where she sank down on the spot on which Cadmus built Thebes, with the acropolis, Cadmea. As he intended to sacrifice the cow here to Athena, he sent some persons to the neighbouring well of Ares to fetch water. This well was guarded by a dragon, a son of Ares, who killed the men sent by Cadmus. Hereupon, Cadmus slew the dragon, and, on the advice of Athena, sowed the teeth of the monster, out of which armed men grew up, who slew each other, with the exception of five, Echion, Udaeus, Chthonius, Hyperenor, and Pelor, who, according to the Theban legend, were the ancestors of the Thebans. Cadmus was punished for having slain the dragon by being obliged to serve for a certain period of time, some say one year, others eight years. After this Athena assigned to him the government of Thebes, and Zeus gave him Harmonia for his wife. The marriage solemnity was honoured by the presence of all the Olympian gods in the Cadmea. Cadmus gave to Harmonia the famous peplos and necklace which he had received from Hephaestus or from Europa, and became by her the father of Autonoë, Ino, Semele, Agave, and Polydorus. Subsequently Cadmus and Harmonia quitted Thebes, and went to the Cenchelians This people was at war with the Illyrians, and had received an oracle which promised them victory if they took Cadmus as their commander. The Cenchelians accordingly made Cadmus their king, and conquered the enemy. After this, Cadmus had another son, whom he called Illyrius. In the end, Cadmus and Harmonia were changed into dragons, and were removed by Zeus to Elysium.

This is the account given by Apollodorus (iii. 1. § 1, &c.), which, with the exception of some particulars, agrees with the stories in Hyginus (Fab. 178) and Pausanias (ix. 5. § 1, 10. § 1, 12. § 1,&c.). There are, however, many points in the story of Cadmus in which the various traditions present considerable differences. His native country is commonly stated to have been Phoenicia, as in Apollodorus (comp. Diod. iv. 2; Strab. vii. p. 321, ix. p. 401); but he is sometimes called a Tyrian (Herod. ii. 49; Eurip. Phoen. 639), and sometimes a Sidonian. (Eurip. Bacch. 171; Ov. Met. iv. 571.) Others regarded Cadmus as a native of Thebes in Egypt (Diod. i. 23; Paus. ix. 12. § 2), and his parentage is modified accordingly; for he is also called a son of Antiope, the daughter of Belus, or of Argiope, the daughter of Neilus. (Schol. ad Eurip. Phoen. 5, with Valck. note; Hygin. Fab. 6, 178, 179.) He is said to have introduced into Greece from Phoenicia or Egypt an alphabet of sixteen letters (Herod. v. 58, &c.; Diod. iii. 67, v. 57; Plin. H. N. vii. 56; Hygin. Fab. 277), and to have been the first who worked the mines of mount Pangaeon in Thrace. The teeth of the dragon whom Cadmus slew were sown, according to some accounts, by Athena herself; and the spot where this was done was shown, in aftertimes, in the neighbourhood of Thebes. (Schol. ad Eurip. Phoen. 670; Paus. ix. 10. § 1.) Half of the teeth were given by Athena to Aeëtes, king of Colchis. (Apollon. Rhod. iii. 1183; Apollod. i. 9. § 23; Serv. ad Virg. Georg. ii. 141.) The account of his quitting Thebes also was not the same in all traditions; for some related, that he was expelled by Amphion and Zethus, or by Dionysus. (Syncell. p. 296, ed. Dindorf.) A tradition of Brasiae stated, that Cadmus, after discovering the birth of Dionysus by his daughter Semele, shut up the mother and her child in a chest, and threw them into the sea. (Paus. iii. 24. § 3.) According to the opinion of Herodotus (ii. 49), however, Melampus learned and received the worship of Dionysus from Cadmus, and other traditions too represent Cadmus as worshipping Dionysus. (e.g. Eurip. Bacch. 181.) According to Euripides, Cadmus resigned the government of Thebes to his grandson, Pentheus; and after the death of the latter, Cadmus went to Illyria, where he built Buthoë (Bacch. 43, 1331, &c.), in the government of which he was succeeded by his son Illyrius or Polydorus.

The whole story of Cadmus, with its manifold poetical embellislinients, seems to suggest the immigration of a Phoenician or Egyptian colony into Greece, by means of which civilisation (the alphabet, art of mining, and the worship of Dionysus) came into the country. But the opinion formed on this point must depend upon the view we take of the early influence of Phoenicia and Egypt in general upon the early civilisation of Greece. While Buttmann and Creuzer admit such an influence, C. O. Muller denies it altogether, and regards Cadmus as a Pelasgian divinity. Cadmus was worshipped in various parts of Greece, and at Sparta he had a heroum. (Paus. iii. 15. § 6.)