The award, put up Thursday by Microsoft, is the third so far as part of a $5 million program the software company announced in early November to help identify the authors of unusually damaging Internet infections that target the company's products.
"This worm is a criminal attack," said Brad Smith, Microsoft's senior VP and general counsel. "Microsoft wants to help the authorities catch this criminal."
The MyDoom.B virus, spread by E-mail, causes victims to launch an electronic attack starting Tuesday against Microsoft's own Web site, and prevents victims from visiting the Web sites of leading antivirus companies. The virus poses as an authentic-looking error message.
Among the only clues to the identity of the possible author was a mysterious message inside the virus, "Andy; I'm just doing my job, nothing personal, sorry."
Microsoft urged anyone with information about the author of the MyDoom.B virus to contact the FBI, Secret Service, or Interpol.
Government officials and others have described the $250,000 rewards as the highest in recent memory funded entirely by the private sector.
Microsoft said residents of any country are eligible for the $250,000. The company has said previously it will not pay rewards to anyone involved in creating the viruses.
Previous rewards of $250,000 each were offered for information about those responsible for the Blaster and Sobig viruses, which spread rapidly last summer among hundreds of thousands of computers running Windows.