SAN FRANCISCO — Enough with the tweets, the texts, the blogs, the Internet searches.

That's the message being communicated by courts across the country as jurors using their portable electronic devices continue to cause mistrials, overturned convictions and chaotic delays in court proceedings.

Last year a San Francisco Superior Court judge dismissed 600 potential jurors after several acknowledged going online to research the criminal case before them.

Baltimore Mayor Sheila Dixon challenged her misdemeanor embezzlement conviction after discovering five jurors "friended" one another on Facebook during the trial.

And a federal judge in Florida declared a mistrial after eight jurors admitted Web surfing about a drug case.

But the rules for jury service in state and federal courts alike are evolving to grapple with this 21st century issue. New jury instructions are being adopted and electronics are being banned from courtrooms.

In January, the federal court's top administrative office, the Judicial Conference of the United States, issued so-called "Twitter instructions" to every federal judge, which are designed to be read to jurors at the start of the trial and before deliberations.

"You may not use any electronic device or media" in connection with the case, the recommended federal instructions admonish. They also bar visits to "any Internet chat room, blog, or website such as Facebook, My Space, LinkedIn, YouTube or Twitter."

The guidelines were developed "to address the increasing incidence of juror use of such devices as cellular telephones or computers to conduct research on the Internet or communicate with others about cases," according to a memo to federal judges from the committee's chief, U.S. District Court Judge Julie Robinson of Topeka, Kan.

"Such use," the judge noted, "has resulted in mistrials, exclusion of jurors, and imposition of fines."

While federal judges can ignore those guidelines, some state judges are not so free.

The Michigan Supreme Court ordered judges there starting Sept. 1 to order jurors to refrain from using cell phones, computers and other electronic devices to discuss cases before them.

San Francisco Superior Court on Jan. 1 began including such instructions after some of the 600 jurors said they went online because there were no explicit prohibitions against such independent research.

"You may not do research about any issues involved in the case," the new instruction states. "You may not blog, Tweet, or use the Internet to obtain or share information."

A California legislator last month introduced a bill that would charge wayward jurors with a crime.

Several courts from Fort Wayne, Ind. to sparsely populated Malheur County in eastern Oregon have gone so far as to completely ban electronic devices.

After electronic communications caused two mistrials, St. Paul, Minn., residents called to jury duty are now warned: "DO NOT BRING WIRELESS COMMUNICATION DEVICES: PHONES, PAGERS, AND PDA'S. Phones are available in the Jury Assembly Room."

The issue first surfaced a few years ago, but has only in the last few months garnered widespread attention because of the increased number of high-profile and disruptive incidents.

"Everyone now has the technology," said New York media lawyer Eric Robinson, who blogs about the issue for Harvard University's Citizen Law Media Project Web site. He said so many people obsessively "tweet" throughout the day that it has almost become an unconscious action.

Robinson said until such behavior is labeled as bad etiquette, tweeting and blogging jurors will continue to frustrate judges.

He said two appeals courts in Maryland and one in New Jersey had so far reversed criminal convictions because of jurors use of technology.

While federal judges hope the new jury instructions will significantly limit jury problems, the National Center for State Courts in Williamsburg, Va., says state judges continue to grapple with how best to deal with the issue.

"The thing that makes the electronic media issue a little different is that it so accessible and anonymous," said Greg Hurley, an analyst at the center. "Jurors face exposure if they go to the library or drive by a crime scene — but there's little risk in going online."