Roche Holding AG confirmed a patient with H1N1 influenza in Denmark showed resistance to the antiviral drug.
David Reddy, company executive, said it was not unexpected given that common seasonal flu could do the same.
The news comes as a nine-year-old girl has become the third to die in the UK with swine flu.
Virologist Professor John Oxford told the BBC, “I’m not surprised about this finding. The question is whether it is going to spread. We will soon know the answer.”