An employee of America Online has been for stealing AOL user screen names and selling them to a spam e-mail operation.
AOL said it discovered the screen name thefts and passed the information on to federal law enforcement agencies, leading to the arrest. AOL has fired the employee, it said, adding it is committed to his full prosecution.
No member credit card or password information was compromised, AOL said.
The number of screen names stolen totalled 92 million. The e-mail spammer had also been arrested. The AOL employee received over $100,000 (£55,000) for the list of names.
The pair face up to five years in jail and heavy fines if convicted under the US Controlling the Assault of Non-Solicited Pornography and Marketing (Can-Spam) Act, which came into effect at the beginning of this year.
AOL said it is reviewing and strengthening its internal procedures as a result of this investigation and arrest.
The incident comes the day after AOL, Yahoo, Microsoft and EarthLink formed the Anti-Spam Technical Alliance which released a set of recommendations for eliminating spam.
David Legard writes for IDG News Service