Revealing the frequency and determinants of reassortment among RNA genome segments is fundamental to understanding basic aspects of the biology and evolution of the influenza virus. To estimate the extent of genomic reassortment in influenza viruses circulating in North American swine, we performed a phylogenetic analysis of 139 whole-genome viral sequences sampled during 1998-2011 and representing seven antigenically distinct viral lineages. The highest amounts of reassortment were detected between the H3 and the internal gene segments (PB2, PB1, PA, NP, M, and NS), while the lowest reassortment frequencies were observed among the H1γ, H1pdm, and NA segments, particularly N1. Less reassortment was observed among specific HA-NA combinations that were more prevalent in swine, suggesting that some genome constellations may be evolutionarily more stable.