He said the finding was no reason to panic.

The drug in question, Tamiflu, still attacks "the vast majority of the viruses out there," said Yoshihiro Kawaoka of the University of Tokyo and the University of Wisconsin-Madison. The drug, produced by Swiss-based Roche Holding AG, is in short supply as nations around the world try to stock up on it in case of a global flu pandemic.

Mr. Kawaoka said the case of resistant virus in the 14-year-old girl is "only one case, and whether that condition was something unique we don't know."

In lab tests, the girl's Tamiflu-resistant virus was susceptible to another drug, Relenza, which is made by GlaxoSmithKline.

Kawaoka and colleagues report the case in the Oct. 20 issue of the journal Nature, which released the study Friday. The researchers conclude that it might be useful to stockpile Relenza as well as Tamiflu.

There's no evidence that so-called H5N1 viruses — like the one recovered from the girl — are becoming generally resistant to the class of drugs that includes Tamiflu, the federal Centers for Disease Control and Prevention said Friday.

Both Tamiflu and Relenza are being stockpiled by the U.S. government. Doctors have good reason to believe Tamiflu would be effective at combatting a pandemic strain of bird flu, although it's not clear how long people would have to be treated or what doses they'd need, said Dr. John Treanor of the University of Rochester.

In adults with mild cases of ordinary flu, Tamiflu speeds up recovery by a day or two, he said. Its effect on severe flu like bird flu isn't clear, he said.

The new report, while not surprising, shows scientists must find out more about how people with bird flu respond to Tamiflu and how often they shed drug-resistant virus, he said.

The shed virus could become a problem if it is transmitted to other people, he said. He noted that in the new report, as in prior studies, the resistant virus was less able to reproduce itself than normal virus was, which might cut down on the chance of transmission.

The girl, who had been caring for an older brother with the disease, had been taking low doses of Tamiflu as a preventive measure when the virus was isolated in late February. She later got sick and was given higher doses. She recovered and left the hospital in March.

Mr. Kawaoka said it's not clear whether the low preventive dose had encouraged the emergence of drug resistance.

Dr. William Schaffner of Vanderbilt University called the report important and said it shows the importance of watching for drug resistance.

"It is not unusual to find the occasional resistant virus," he said. "It could be just a biological oddity, or we could see this more frequently.

"This is a blip on the radar screen, and it surely does mean that we have to keep the radar operative," Dr. Schaffner said. "We have to keep testing more viruses."

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