It's the main drug being stockpiled in case of a bird flu pandemic, but researchers are saying it's important to have a second drug on hand.
They're pointing to the case of a Vietnamese girl who infected with the virus, and showed resistance to Tamiflu.
The lead researcher, Yoshihiro Kawaoka of the University of Tokyo and the University of Wisconsin-Madison, says the finding isn't reason for panic, saying Tamiflu still attacks "the vast majority of the viruses out there." And he says some strains of human flu have also showed resistance to Tamiflu.
The girl's virus was susceptible to another drug, Relenza. The U.S. is stockpiling both Tamiflu and Relenza in case of a pandemic.
The research is in the journal Nature.