Having tested the ability of the method at reconstructing transmission trees from outbreak genetic data, we used SeqTrack to infer the spatiotemporal dynamics of the early stage of the 2009 swine-origin A/H1N1 influenza pandemic. The data set analyzed consisted of 433 nearcomplete HA and NA sequences collected between 30 March and 12 July 2009. Although no sequences are available for the first confirmed cases (La Gloria, 15 February; Fraser et al., 2009) , the time window defined by the collection dates extends from the early stages of the outbreak to the global pandemic, which was officially declared by the WHO on 11 June 2009. In Figure 5 , we present snapshots of the spatiotemporal dynamics of the pandemic reconstructed by SeqTrack. Overall, most inferred ancestries displayed very few mutations between ancestor and descendent (Supplementary Figure  S3) , suggesting that the actual ancestral haplotype had often been sampled, and was correctly identified by the method. The inferred scenario of the early stages of the pandemic can be split into three main acts: (1) initial spread in Mexico and the United States (Figure 5a ), (2) sustained transmission within North America and spread to the rest of the world (Figure 5b) and (3) further worldwide spread and secondary outbreaks outside the Americas (Figure 5c ). For a day-by-day reconstruction of the pandemic, see Supplementary Movie S1 and Supplementary Information. Details of inferred ancestries are provided in Supplementary Table S2 .