Transit authorities in 2003 launched the “See Something, Say Something” campaign to help make riders active participants in policing the subway.

Now, the Metropolitan Transportation Authority has filed a trademark application in order to police the phrase and those who use it. The move has surprised officials at many transit agencies, who had assumed the phrase, which is now used in various iterations as part of transit security programs from San Francisco to Sydney, Australia, was an industry version of a free download.

“It was so out there that it was a public domain kind of thing,” a spokeswoman for San Francisco’s Golden Gate Bridge, Highway and Transportation District Mary Currie, said, “We just assumed it was something we could use.”

Ms. Currie said transit officials there unveiled a program last week called “See Something? Say Something!” for the bridges, ferries, and buses they operate.

Ms. Currie and her organization have little to worry about, a spokesman for the MTA, Tom Kelly, said. The MTA, which filed its application in August but will have to wait until March before it is reviewed by the U.S. Patent and Trademark Office, is pursuing the trademark to ensure the phrase would always be used to promote transit security, though not necessarily within the MTA’s bridges, tunnels, subways, and railroads.

Asked to give an example of an inappropriate use, Mr. Kelly said: “If you see a bikini-clad woman with the ‘see something, say something’ phrase on her, that would be an inappropriate use of it.”

The slogan, though, has already moved into areas that blur the lines of its intended use. A German publisher, Hatje Cantz, released a book last year on a French photographer, Touhami Ennadre, titled “Touhami Ennadre: If You See Something, Say Something.” The publisher could not be reached; its American distributor, Distributed Art Publishers, based in Manhattan, declined to comment.

The phrase, and the request it makes of riders, has also reached into the realm of academia. An associate professor at the School of the Art Institute of Chicago, Kevin Tavin, has given a series of lectures and said he is proposing a book that investigates how the widespread use of the phrase and its meaning are part of a “much larger discourse of fear and terror in our everyday lives.” His proposed title: “If You See Something, Say Something: Visual Culture and the War of Terror.”

“Well, if I have colon after it and add something on, I don’t think it violates copyright at all,” he said.

Mr. Tavin came up with the title while riding the elevated subway in Chicago, which has used the “See Something, Say Something” phrase, with the MTA’s permission, since May 2003.

The wide use of the phrase on Boston’s “T” by the Massachusetts Bay Transportation Authority led police at Harvard University to incorporate the phrase in a book on safety for undergraduates.

While MTA officials have made some noises about protecting the phrase from being used for non-transit purposes, a spokesman for Harvard, Joe Wrinn, said a request for permission from the MTA has not yielded a response, though he said the university would gladly make the change if requested.

“It’s a very clever saying,” Mr. Wrinn said. “It really summarizes the kinds of things we’ve been saying for the past few years to publicize our community policing policy.”

The MTA has long fought to protect its trademarked products, from subway maps to train logos. A trademark attorney, Martin Schwimmer, said this trademark application was unusual because the phrase is part of a public service campaign intended for widespread use and reproduction, whereas trademarks are intended to limit use.

“That’s an odd way of going about a public service campaign,” Mr. Schwimmer said. While everybody seems to be using the phrase now, that might not last.

The Washington Metro deployed its “See it? Say it!” campaign earlier this year after deciding its “Excuse me, is that your bag?” slogan had run its course, a spokeswoman, Lisa Farbstein, said.

“This, too, may be altered in another year or so just to freshen it up so people listen to it and don’t tune out.