We assayed the temperature sensitivity of 2009 pandemic influenza A(H1N1) viral isolates (n=23) and seasonal influenza A(H1N1) viruses (n=18) isolated in northern France in 2007/08 and 2008/09. All isolates replicated with a similar efficiency at 34 °C and 37 °C, and with a lower efficiency at 40 °C. The pandemic viral isolates showed a stronger heterogeneity in their ability to grow at the highest temperature, as compared with the seasonal isolates. No statistically significant difference in temperature sensitivity was observed between the pandemic viral isolates from severe and mild cases of influenza. Our data point to the impact of temperature sensitivity on the genetic evolution and diversification of the pandemic influenza A(H1N1) virus since its introduction into the human population in April 2009, and call for close surveillance of this phenotypic marker related to host and tissue tropism.