A spokeswoman for the Internet portal said that an individual or individuals posing as part of Yahoo! had sent e-mails to users in an effort to trick them into disclosing their online account information.
The spokeswoman said Yahoo! "takes all reports of fraud by third parties very seriously" and has alerted its users to the scam.
According to reports, less than 24 hours elapsed between the time the bogus e-mail was sent and the time Yahoo! sent out its own mass e-mail to its users advising them not to respond to the request.
It was not immediately clear what the fraudulent e-mail to customers said.
A Yahoo! spokeswoman said that although some users did divulge personal information, most users did not.
Yahoo! also said it didn't know the origin of the fraudulent e-mail.
Last month, online payment service PayPal was also targeted by scam artists trying to get the personal information of its users. However, unlike Yahoo!, PayPal did not notify its customers of the scam.