The fine was imposed by the Commodity Futures Trading Commission (CFTC) which found the two companies gave false trading information on natural gas to certain reporting firms.
The CFTC settlement order found that "Dynegy knowingly submitted false information to the reporting firms in an attempt to skew (gas price) indexes for Dynegy's financial benefit."
In a statement Dynegy said it had agreed to pay the fine without admitting or denying the CFTC's charges.
Over the past year, Dynegy has seen its share price slump and has faced severe financial difficulties. Last month, it became the latest in a string of companies to receive a subpoena in the investigation into California's energy trading business.
The investigation is examining the trading practises of energy companies during the energy crisis of late 2000 to 2001, when wholesale electricity prices soared.
