In spring 334 BC, as a young man of about twenty-three, Seleucus accompanied Alexander into Asia.
By the time of the Indian campaigns beginning in late in 327 BC, he had risen to the command of the élite infantry corps in the Macedonian army, the "Shield-bearers" (Hypaspistai, later known as the "Silvershields").
It is said by Arrian that when Alexander crossed the Hydaspes river on a boat, he was accompanied by Perdiccas, Ptolemy I Soter, Lysimachus and also Seleucus.
During the subsequent Battle of the Hydaspes (326 BC), Seleucus led his troops against the elephants of King Porus.
It is unknown the extent in which Seleucus participated in the actual planning of the battle, as he is not mentioned as holding any major independent position during the battle.
This contrasts Craterus, Hephaistion, Peithon and Leonnatus – each of whom had sizable detachments under his control.
Seleucus' Royal Hypaspistai were constantly under Alexander's eye and at his disposal.
They later participated in the Indus Valley campaign, in the battles fought against the Malli and in the crossing of the Gedrosian desert.

At the great marriage ceremony at Susa in the spring of 324 BC, Seleucus married Apama (daughter of Spitamenes), and she bore him his eldest son and successor Antiochus I Soter, at least two legitimate daughters (Laodice and Apama) and possibly another son (Achaeus).
At the same event, Alexander married the daughter of the late Persian King Darius III while several other Macedonians married Persian women.
After Alexander's death (323 BC), when the other senior Macedonian officers unloaded their "Susa wives" en masse, Seleucus was one of the very few who kept theirs, and Apama remained his consort (later Queen) for the rest of her life.

Ancient sources  mention Seleucus three times before the death of Alexander.
He participated in a sailing trip near Babylon, took part in the dinner party of Medeios the Thessalian with Alexander and visited the temple of the god Serapis.
In the first of these episodes, Alexander's diadem was blown off his head and landed on some reeds near the tombs of Assyrian kings.
Seleucus swam to fetch the diadem back, placing it on his own head while returning to the boat to keep it dry.
The validity of the story is dubious.
The story of the dinner party of Medeios may be true, but the plot to poison the King is unlikely.
In the final story, Seleucus reportedly slept in the temple of Serapis in the hope that Alexander's health might improve.
The validity of this story is also questionable, as the Graeco-Egyptian Serapis had not been invented at the time.
In spring 334 BC, as a young man of about twenty-three, Seleucus accompanied Alexander into Asia. By the time of the Indian campaigns beginning in late in 327 BC, he had risen to the command of the élite infantry corps in the Macedonian army, the "Shield-bearers" (Hypaspistai, later known as the "Silvershields"). It is said by Arrian that when Alexander crossed the Hydaspes river on a boat, he was accompanied by Perdiccas, Ptolemy I Soter, Lysimachus and also Seleucus. During the subsequent Battle of the Hydaspes (326 BC), Seleucus led his troops against the elephants of King Porus. It is unknown the extent in which Seleucus participated in the actual planning of the battle, as he is not mentioned as holding any major independent position during the battle. This contrasts Craterus, Hephaistion, Peithon and Leonnatus – each of whom had sizable detachments under his control. Seleucus' Royal Hypaspistai were constantly under Alexander's eye and at his disposal. They later participated in the Indus Valley campaign, in the battles fought against the Malli and in the crossing of the Gedrosian desert.
At the great marriage ceremony at Susa in the spring of 324 BC, Seleucus married Apama (daughter of Spitamenes), and she bore him his eldest son and successor Antiochus I Soter, at least two legitimate daughters (Laodice and Apama) and possibly another son (Achaeus). At the same event, Alexander married the daughter of the late Persian King Darius III while several other Macedonians married Persian women. After Alexander's death (323 BC), when the other senior Macedonian officers unloaded their "Susa wives" en masse, Seleucus was one of the very few who kept theirs, and Apama remained his consort (later Queen) for the rest of her life.
Ancient sources  mention Seleucus three times before the death of Alexander. He participated in a sailing trip near Babylon, took part in the dinner party of Medeios the Thessalian with Alexander and visited the temple of the god Serapis. In the first of these episodes, Alexander's diadem was blown off his head and landed on some reeds near the tombs of Assyrian kings. Seleucus swam to fetch the diadem back, placing it on his own head while returning to the boat to keep it dry. The validity of the story is dubious. The story of the dinner party of Medeios may be true, but the plot to poison the King is unlikely. In the final story, Seleucus reportedly slept in the temple of Serapis in the hope that Alexander's health might improve. The validity of this story is also questionable, as the Graeco-Egyptian Serapis had not been invented at the time.
Seleucus I Nicator