Comcast Corp. CEO Brian Roberts will pay a $500,000 civil penalty for failing to report to the federal government his purchases of Comcast stock that were required under Hart-Scott-Rodino antitrust rules.

The Federal Trade Commission and U.S. Justice Department said on Friday that not informing the government of the stock purchases between 2007 and 2009 was a mistake and the apparent result of bad outside legal advice, but it also was Roberts's third violation - though the previous two did not lead to fines.

The penalty disclosed Friday resulted when Roberts acquired cable company shares through his compensation package and 401(k) plan without filing the necessary paperwork with the two government agencies.

Roberts wasn't fined in 1999 and 2000 for violations related to the Internet Capital Group Inc. and Susquehanna Cable transactions.

"He had his free bite at the apple but he did it again," said Barry Reingold, antitrust partner at Perkins Coie L.L.P. in Washington.

Reingold believed the penalty was modest because Roberts faced fines of between $11,000 and $16,000 a day for continuous violations over almost two years - which could have amounted to more than $8 million, based on the government's description of the maximum daily levies.

"He didn't make a killing and he got bad legal advice," Reingold said of the government's treatment of the cable executive.

A handful of Hart-Scott-Rodino penalties are levied against executives each year, according to an official at the Federal Trade Commission, which investigated Roberts. Media executive John Malone paid $1.4 million in 2009. Microsoft founder Bill Gates paid $800,000 in 2004.

Many chief executives don't believe they have to file information on stock purchases under Hart-Scott-Rodino, which was established in the 1970s to provide the Federal Trade Commission and the Justice Department with advance information about mergers and acquisitions, so they can be evaluated for anti-competition concerns.

Those government agencies then can block a transaction, as they did recently in the proposed AT&T/T-Mobile wireless merger, if they believe it would harm consumers with higher prices or lead to anti-competitive conditions in an industry.

Roberts filed information under Hart-Scott-Rodino in 2002 when Comcast purchased AT&T Corp.'s cable business. But he was required to file again five years later, which he failed to do.

Government officials require the five-year reporting cycle to see how much stock an executive has acquired and whether this could be an antitrust issue. Though Roberts's stock purchases weren't likely to result in anti-competitive issues, he still was required under government rules to file the information.

According to a Justice Department complaint, Roberts acquired shares in Comcast through his compensation package and 401(k) retirement plan.

Roberts voluntarily told the government in 2009 about the violation. He also had told the government about the violations in 1999 and 2000.

Comcast said in a statement: "Comcast and Mr. Roberts appreciate the acknowledgement by the Federal Trade Commission that this was a technical and inadvertent violation that was self reported, promptly corrected, and did not involve any financial gain to the Company or to Mr. Roberts. We take very seriously our obligations to comply with all aspects of the Hart-Scott-Rodino Act and working with our lawyers we have put in place additional safeguards to ensure that an inadvertent violation does not occur in the future."

Roberts has agreed to pay the $500,000 within 30 days.