IMGT/PhyloGene is an on-line software package for comparative analysis of immunoglobulin (IG) and T cell receptor (TR) variable genes of all vertebrate species, newly implemented in IMGT, the international ImMunoGeneTics information system ((R)). IMGT/PhyloGene is strongly associated with the IMGT gene and allele nomenclature and with the IMGT unique numbering for V-REGION, which directly creates standardized alignments from IMGT reference sequences. IMGT/PhyloGene is the first tool to use the IMGT expertized and standardized data for automated comparative analyses, and the first on-line software package for phylogenetic reconstruction to be integrated to a sequence database. Starting from a standardized alignment of selected sequences, IMGT/PhyloGene computes a matrix of evolutionary distances, builds a tree using the Neighbor-Joining (NJ) algorithm, and outputs various graphical tree representations. The resulting IMGT/PhyloGene tree is then used as a support for studying the evolution of particular subregions, such as the CDR-IMGT (Complementarity Determining Regions) or the V-RS (Variable gene Recombination Signals). IMGT/PhyloGene is freely available at http://imgt.cines.fr.