The U.S. Internal Revenue Service is asking Symantec to pay $900 million in back taxes from last year's $10.25 billion acquisition of storage software vendor Veritas, the Cupertino, Calif.-based vendor revealed today.

Symantec also said the IRS is seeking approximately $100 million, excluding penalties and interest, from an unrelated audit of the vendor for fiscal years 2003 and 2004.

In a Securities and Exchange Commission filing, Symantec indicated it believes the IRS's claims are out of line with current tax laws and Treasury regulations, and plans to file a petition with the tax court to challenge the assessment.

Symantec's shares have now lost nearly a third of their value since the Veritas acquisition, and the company recently announced the departure of three senior executives. Earlier this year, former Veritas CEO Gary Bloom and former Symantec CFO Greg Myers also left the company.

Shares of Symantec fell 66 cents to $15.73 in after hours trading Monday after closing the day at $16.39.