Hypponen said Cabir was found on Monday in a technology gadgets store in Santa Monica, California, when a passing techie spotted a telltale sign on the screen of a phone in the store.
"It's interesting (the Cabir variant) has now been found in the United States, but it's not the end of the world," said Hypponen.
The mobile-virus threat will grow in the future as virus-writers become more sophisticated and phones standardize on technologies that make it easier for viruses to spread across not just specific devices but the whole industry.
The danger is small at the moment, in part because of the range of handheld technologies. This is unlike the personal computer world dominated by the Windows operating system made by Microsoft Corp.
Also, many handheld device makers have recently released new mobile phones equipped with anti-virus software.
The store owner's phone had also been infected, Hypponen said. Both devices were Nokia 6600 smart phones, which combine phone and computer functions, like e-mail. Nokia is the world's biggest mobile phone maker.
Analysts say the various features in smart phones make them more vulnerable to viruses than voice-only phones.
Hypponen said it was likely other devices in the area were also infected by Cabir, although there was no confirmation of that.
Unlike computer viruses that spread quickly around the world via the Internet, Cabir spreads slowly because it travels only over short distances through a wireless technology known as Bluetooth. It also requires a user to restart the phone after it has been exposed for the virus to take hold.
In cases where Cabir spread to different countries, an infected phone has typically been carried by the user to another country.
Cabir has been found in countries ranging from China to the United Kingdom.