According to Wikipedia, the online encyclopedia, John Seigenthaler Sr. is 78 years old and the former editor of The Tennessean in Nashville. But is that information, or anything else in Mr. Seigenthaler's biography, true?

The question arises because Mr. Seigenthaler recently read about himself on Wikipedia and was shocked to learn that he "was thought to have been directly involved in the Kennedy assassinations of both John and his brother Bobby."

Mr. Seigenthaler discovered that the false information had been on the site for several months and that an unknown number of people had read it, and possibly posted it on or linked it to other sites.

If any assassination was going on, Mr. Seigenthaler (who is 78 and did edit The Tennessean) wrote last week in an op-ed article in USA Today, it was of his character.

The case triggered extensive debate on the Internet over the value and reliability of Wikipedia, and more broadly, over the nature of online information.

Wikipedia is a kind of collective brain, a repository of knowledge, maintained on servers in various countries and built by anyone in the world with a computer and an Internet connection who wants to share knowledge about a subject. Literally hundreds of thousands of people have written Wikipedia entries.

Mistakes are expected to be caught and corrected by later contributors and users.

The whole nonprofit enterprise began in January 2001, the brainchild of Jimmy Wales, 39, a former futures and options trader who lives in St. Petersburg, Fla. He said he had hoped to advance the promise of the Internet as a place for sharing information.

It has, by most measures, been a spectacular success. Wikipedia is now the biggest encyclopedia in the history of the world. As of Friday, it was receiving 2.5 billion page views a month, and offering at least 1,000 articles in 82 languages. The number of articles, already close to two million, is growing by 7 percent a month. And Mr. Wales said that traffic doubles every four months.

Still, the question of Wikipedia, as of so much of what you find online, is: Can you trust it?

And beyond reliability, there is the question of accountability. Mr. Seigenthaler, after discovering that he had been defamed, found that his "biographer" was anonymous. He learned that the writer was a customer of BellSouth Internet, but that federal privacy laws shield the identity of Internet customers, even if they disseminate defamatory material. And the laws protect online corporations from libel suits.

He could have filed a lawsuit against BellSouth, he wrote, but only a subpoena would compel BellSouth to reveal the name.

In the end, Mr. Seigenthaler decided against going to court, instead alerting the public, through his article, "that Wikipedia is a flawed and irresponsible research tool."