The Federal Communications Commission yesterday moved to prevent cell phones and personal digital assistants (PDAs) from becoming the next major frontier for e-mail spam.

The agency issued rules requiring marketers to have explicit permission from wireless-device users before they can be sent any commercial e-mail. And it urged the industry to develop technologies to prevent spam from overwhelming wireless devices the way it has swamped computer messaging.

"By prohibiting all commercial messages to wireless phones and PDAs absent affirmative consent from the consumer, Americans can now use their wireless devices freely, without being bothered by unwanted and annoying messages," FCC Chairman Michael K. Powell said in a prepared statement.

The rule covers a rapidly expanding array of Web-based e-mail services offered by wireless providers that enable customers to send and read messages via their phones or PDAs.

But it does not apply to services that simply forward existing computer e-mail messages to a wireless device or allow the wireless device to connect to a computer-based mail account so that e-mail can be read.

Another type of communication, phone-to-phone text messaging, is not covered by the rule issued yesterday, but the FCC said such messages generated automatically by a computer would run afoul of other laws.

The rule stands in contrast to the national anti-spam law passed by Congress last year, which allows marketers to send unsolicited e-mail until they are asked by consumers to stop.

To help marketers avoid mistakenly sending e-mail to wireless-system users, carriers will make available the family of Internet addresses they use on their systems, known as domains. Individual addresses will not be collected.