Ex-Enron chief Jeffrey Skilling said he never thought the firm he helped build would collapse, as he proclaimed his innocence on the witness stand.
Mr Skilling's testimony could make or break his defence, experts say
Along with Enron founder Kenneth Lay, he is charged with hiding billions of dollars of losses and of lying about the state of the energy-trading firm.
He told the court that he had never committed any crime, and accounting at Enron had been properly carried out.
The firm collapsed in 2001 with debts of $31.8bn (£18bn).
Mr Skilling's testimony kicked off the 11th week of the trial, and experts have said it could make or break his defence case.
He is charged with 28 counts of conspiracy and fraud related to the company's collapse.
Meanwhile Mr Lay, 63, faces six charges of fraud and conspiracy. Both men deny the charges.
Mr Skilling admitted he was nervous as the questioning began.
"I guess in some ways my life is on the line," he said.
After telling the federal court in Houston that he was "absolutely innocent", he vowed to "fight those charges till the day I die".
He told jurors that the firm was "in very good condition" when he left in August 2001.
His lawyer, Daniel Petrocelli, also asked whether Mr Skilling had any idea that the company was on the brink of a huge financial scandal.
"Not in my widest dreams, no. It's almost inconceivable now what happened," Mr Skilling told the court.
His decision to spend more time with his family had prompted his decision to resign, he said, but he also conceded that he had told Mr Lay he wanted to quit as he was concerned about Enron's falling stock price.
'Crisis of confidence'
Mr Skilling said Enron had collapsed because of a lack of market confidence and a "run on the bank" situation, when creditors called in loans and the firm did not have enough money to finance itself.
But as news of its problems emerged, Mr Skilling said he offered to return to the firm, but was rebuffed.
"By coming back, I would send a strong signal to the marketplace that I didn't think anything was wrong," he said.
The former Enron boss also consistently denied destroying documents and computers, setting up offshore accounts to hide money or trying to cover-up past behaviour or dealings.
He also voiced the belief that the "vast majority" of former Enron colleagues who had pleaded guilty to crimes associated with the firm's collapse were in fact innocent.
During the trial, the defence has consistently argued that these individuals who cooperated with the prosecution were actually innocent and had been pushed into making guilty pleas to avoid trials or long prison sentences.
Both he and Mr Lay have laid the blame for the accounting problems at the door of Enron's former chief financial officer Andrew Fastow. Mr Fastow has already pleaded guilty to fraud charges and is a prosecution witness.