Last year a Wikipedia visitor edited the entry for the SeaWorld theme parks to change all mentions of “orcas” to “killer whales,” insisting that this was a more accurate name for the species.

There was another, unexplained edit: a paragraph about criticism of SeaWorld’s “lack of respect toward its orcas” disappeared. Both changes, it turns out, originated at a computer at Anheuser-Busch, SeaWorld’s owner.

Dozens of similar examples of insider editing came to light last week through WikiScanner, a new Web site that traces the source of millions of changes to Wikipedia, the popular online encyclopedia that anyone can edit.

The site, wikiscanner.virgil.gr, created by a computer science graduate student, cross-references an edited entry on Wikipedia with the owner of the computer network where the change originated, using the Internet protocol address of the editor’s network. The address information was already available on Wikipedia, but the new site makes it much easier to connect those numbers with the names of network owners.

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Since Wired News first wrote about WikiScanner last week, Internet users have spotted plenty of interesting changes to Wikipedia by people at nonprofit groups and government entities like the Central Intelligence Agency. Many of the most obviously self-interested edits have come from corporate networks.

Last year, someone at PepsiCo deleted several paragraphs of the Pepsi entry that focused on its detrimental health effects. In 2005, someone using a computer at Diebold deleted paragraphs that criticized the company’s electronic voting machines. That same year, someone inside Wal-Mart Stores changed an entry about employee compensation.

Jimmy Wales, founder of the Wikimedia Foundation, which runs Wikipedia, says the site discourages such “conflict of interest” editing. “We don’t make it an absolute rule,” he said, “but it’s definitely a guideline.”

Internet experts, for the most part, have welcomed WikiScanner. “I’m very glad that this has been exposed,” said Susan P. Crawford, a visiting professor at the University of Michigan Law School. “Wikipedia is a reliable first stop for getting information about a huge variety of things, and it shouldn’t be manipulated as a public relations arm of major companies.”

Most of the corporate revisions did not stay posted for long. Many Wikipedia entries are in a constant state of flux as they are edited and re-edited, and the site’s many regular volunteers and administrators tend to keep an eye out for bias.

In general, changes to a Wikipedia page cannot be traced to an individual, only to the owner of a particular network. In 2004, someone using a computer at ExxonMobil made substantial changes to a description of the 1989 Exxon Valdez oil spill in Alaska, playing down its impact on the area’s wildlife and casting a positive light on compensation payments the company had made to victims of the spill.

Gantt Walton, a spokesman for the company, said that although the revisions appeared to have come from an ExxonMobil computer, the company has more than 80,000 employees around the world, making it “more than a difficult task” to figure out who made the changes.

Mr. Walton said ExxonMobil employees “are not authorized to update Wikipedia with company computers without company endorsement.” The company’s preferred approach, he said, would be to use Wikipedia’s “talk” pages, a forum for discussing Wikipedia entries.

Mr. Wales also said the “talk” pages are where Wikipedia encourages editors with a conflict of interest to suggest revisions.

“If someone sees a simple factual error about their company, we really don’t mind if they go in and edit,” he said. But if a revision is likely to be controversial, he added, “the best thing to do is log in, go to the ‘talk’ page, identify yourself openly, and say, ‘I’m the communications person from such and such company.’ The community responds very well, especially if the person isn’t combative.”

Mike Sitrick, a longtime public relations consultant in Los Angeles, agreed. “I’m a big believer that if you’re going to correct it, correct it with a name,” he said. “Otherwise it hurts your credibility.”

An Anheuser-Busch employee eventually took responsibility for the changes to the SeaWorld page — but only after being challenged about them twice by another user. A person identifying himself as Fred Jacobs, communications director for the company’s theme park unit, said on the entry’s “talk” page that discussion of the ethics of keeping sea creatures captive “belongs in an article devoted to that subject.”

Mr. Jacobs referred questions about the editing to another company office, which did not respond to requests for comment.

The SCO Group, a software maker in Salt Lake City, made changes to product information in its own entry this year. The company has been involved in legal disputes over the rights to some open-source software.

Craig Bushman, the company’s vice president for marketing, said he had told a public relations manager to make the changes. “The whole history of SCO had been written by someone who doesn’t know the history of SCO,” he said.

An hour after the changes were made, he said, they disappeared. The company e-mailed Wikipedia administrators, who replied that the changes had been rejected because of a lack of objectivity.

In the case of the Wal-Mart revisions, David Tovar, a company spokesman, said that while he was not aware of anyone within Wal-Mart who had asked to contribute to Wikipedia, the changes could have been made by any of its workers, who are called associates. “We consider our associates our best ambassadors,” he said, “and sometimes they speak out to set the record straight.”

At Dell, the computer maker, employees are told that they need to identify their employer if they write about the company online. “Whether it’s Wikipedia, Twitter or MySpace, our policy is you have to let someone know you’re from Dell,” said Bob Pearson, a Dell spokesman.

Before that policy was put in place a year ago, changes to parts of Dell’s Wikipedia entry discussing its offshore outsourcing of customer service were made by someone from the Dell corporate network.

Most people using company networks to edit Wikipedia entries dabble in subjects that appear to have little to do with their work, although sometimes they cannot resist a silly dig at the competition.

Last year, someone using a computer at the Washington Post Company changed the name of the owner of a free local paper, The Washington Examiner, from Philip Anschutz to Charles Manson. A person using a computer at CBS updated the page on Wolf Blitzer of CNN to add that his real name was Irving Federman. (It is actually Wolf Blitzer.)

And The New York Times Company is among those whose employees have made, among hundreds of innocuous changes, a handful of questionable edits. A change to the page on President Bush, for instance, repeated the word “jerk” 12 times. And in the entry for Condoleezza Rice, the secretary of state, the word “pianist” was changed to “penis.”

“It’s impossible to determine who did any of these things,” said Craig R. Whitney, the standards editor of The Times. “But you can only shake your head when you see what was done to the George Bush and Condoleezza Rice entries.”

WikiScanner is the work of Virgil Griffith, 24, a cognitive scientist who is a visiting researcher at the Santa Fe Institute in New Mexico. Mr. Griffith, who spent two weeks this summer writing the software for the site, said he got interested in creating such a tool last year after hearing of members of Congress who were editing their own entries.

Mr. Griffith said he “was expecting a few people to get nailed pretty hard” after his service became public. “The yield, in terms of public relations disasters, is about what I expected.”

Mr. Griffith, who also likes to refer to himself as a “disruptive technologist,” said he was certain any more examples of self-interested editing would come out in the next few weeks, “because the data set is just so huge.”

Mr. Wales, who called the scanner “a very clever idea,” said he was considering some changes to Wikipedia to help visitors better understand what information is recorded about them.

“When someone clicks on ‘edit,’ it would be interesting if we could say, ‘Hi, thank you for editing. We see you’re logged in from The New York Times. Keep in mind that we know that, and it’s public information,’ ” he said. “That might make them stop and think.”