Jonah was swallowed by a fish because he disobeyed God. In the book of Jonah in the Bible, God instructed Jonah to go and preach repentance to the city of Nineveh, but Jonah disobeyed and tried to run away from God's presence by boarding a ship to Tarshish. God sent a storm, and the sailors threw Jonah overboard, where he was swallowed by a great fish. Jonah remained in the belly of the fish for three days and three nights before he was eventually vomited out on dry land.