Phylogenetic analysis of the virus HA and NA genes indicated that all 12 subtype H5N1 isolates were part of the Vietnam and Thailand lineage (clade 1). The viruses were closely related to those investigated in Thailand (2004) (2005) as well as to other subtype H5N1 isolates in clade 1. In contrast, they differed from infl uenza subtype H5N1 viruses of the south China and Indonesia lineages (clade 2) (Figure 2) . In this study, we did not discern any Thailand isolates closely related to the south China lineage, as previously established in Lao and Cambodia (5) . Phylogenetic analysis of 6 remaining genes showed them to be also closely related to the Vietnam and Thailand isolates.