Clinical use of any antiviral drug can lead to the development of drug-resistant viruses.1,2 Two neuraminidase inhibitors, zanamivir and oseltamivir, are effective against influenza and are used extensively for treatment of this infection, especially in Japan.3,4 We and others have documented the emergence of oseltamivir-resistant type A viruses, including H5N1 subtypes,3,5-7 but similar information on influenza B viruses is limited. Although influenza B viruses usually cause smaller epidemics than type A viruses, they are nonetheless associated with annual outbreaks of illness and excess mortality rates worldwide.8