Genetic data were used to infer the spatiotemporal spread of the early stages of the swine-origin influenza A/H1N1 pandemic. We analyzed all isolates typed for both hemagglutinin (HA) and neuraminidase (NA) genes available from GenBank (Benson et al., 2007;  http://www.ncbi.nlm.nih.gov/Genbank/index.html) as of 20 August 2009. Genetic sequences and their annotation (including sampling dates and locations) were retrieved and processed using ad hoc scripts in the R language. We only retained isolates for which collection date and location were available. Influenza viruses are routinely amplified by serial passaging through cell cultures or chicken eggs before sequencing. Duplicate sequences have been submitted for some isolates that were multiply amplified, sometimes resulting in slightly different sequences for the same isolate. In these cases, significantly shorter sequences were discarded, as well as sequences obtained from amplification in eggs because this method is known to induce additional 'artificial' mutations (Bush et al., 2000) .