Over the last year, the rate of click fraud has risen drastically, reaching the highest rate since measurement began in 2006, according to Click Forensics, a firm that analyzes traffic on behalf of advertisers and ad networks.

Click fraud is the practice of creating dummy Web sites to host online ads, peppering those ads with computer generated-clicks, and then collecting money from unwitting advertisers for those clicks. The clicking is often carried out by “botnets,” or networks of hijacked personal computers, harnessed together by a virus.

Paul Pellman, the chief executive of Click Forensics, said that the firm had begun seeing fraudulent clicks routed through mobile devices, like wireless Internet cards. Such clicks are harder to detect than those coming from wired computers because the wireless card effectively disguises the origin, lumping them in with legitimate mobile users under a single originating address.

“The mobile traffic is getting to be large enough that they can hide within that traffic,” Mr. Pellman said.