Bio::NEXUS package provides an object-oriented, Perl-based applications programming interface (API) to the NEXUS file format of Maddison, et al., 1997 (Syst. Biol. 46:590-621). NEXUS is a powerful and extensible format designed for use in evolutionary analysis, including the analysis of molecular sequence data as well as classical morphological and life-history data. NEXUS is the input or output format for software such as PAUP*, MacClade, Mesquite, SIMMAP, MrBayes, Nexplorer, and so on. This package also contains the demonstration applications nexplot.pl (plot character data with a tree) and nextool.pl (allowing programmatic editing, e.g., selecting particular clades or subsets of data). WWW: http://search.cpan.org/dist/Bio-NEXUS/