A key prosecution witness lied about taking part in Enron-related crimes, an assistant of former Enron chiefs Ken Lay and Jeffrey Skilling has said.
Ms Williamson was the first defence witness in the Enron case
Joannie Williamson told defence lawyers Mark Koenig admitted he had pleaded guilty to crimes he did not commit.
The defence team has repeatedly claimed fear of prosecution had pushed many Enron executives to plead guilty.
Mr Lay and Mr Skilling deny charges of conspiracy, fraud and insider trading linked to the energy firm's collapse.
The US group collapsed in December 2001 after disclosures that it falsified accounts to hide debt and inflate profits.
Mr Skilling now faces 28 cases of fraud, conspiracy and insider trading, while Mr Lay faces six counts of conspiracy and fraud.
Ms Williamson told the court she did not believe Mr Koenig - a close friend and former boss - was guilty even though he had previously told jurors he "pled guilty because I am guilty".
She explained that he had made a telephone call to her on the day he pleaded guilty to aiding and abetting securities fraud and lying about the energy giant's finances.
"I said 'You're not guilty,' and he said 'I know that, but in order for this to work everybody needs to believe that I am'," she told the court in Houston.
Ms Williamson was the first defence witness to take the stand in the case.
Other defence witnesses who faced the court on Monday included former analysts Scott Stoness and Diann Huddleson who worked for Enron's energy retail unit - Enron Energy Services (EES).
Mr Stoness dismissed prosecution claims that in May 2001 Mr Skilling had approved a plan to hide $200m in losses at EES.
According to Mr Stoness, Enron was unaware the unit would make a loss at the division until two months later and that the eventual losses came in at $170m.
Meanwhile, Ms Huddleson denied allegations that EES was in disarray as witnesses for the prosecution had claimed.
Contrary to prosecution claims the firm persistently called on utility firms to pay their bills and also had reserves of up to $400m to cover any unpaid bills.
Lawyers for the defence have said Mr Skilling could offer his side of the case as early as this week.