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.
It is understood from her doctors at Birmingham Children's Hospital that she had underlying health conditions. It is not yet known whether swine flu contributed to her death.
Meanwhile, the Department of Health has announced a big jump in the number of patients in England confirmed with swine flu - up 1,604 since Friday, taking the UK total so far to 5,937.