TreeView is a simple and user-friendly application for viewing phylogenetic trees on personal computers, compatible with both MacOS and Windows. Unlike some phylogeny programs that require data sets to be loaded before trees can be displayed, TreeView allows users to view trees directly. It runs as a native application on both operating systems, supports standard fonts, printers, and native graphics formats (PICT and Windows metafile), and enables drag-and-drop file operations. TreeView can read a variety of tree file formats, including those from CLUSTAL W, COMPONENT, fastDNAml, Hennig86, MacClade, PAUP, PHYLIP, and the Ribosomal Database Project. These formats use a variant of the nested parentheses format, known as Newick or New Hampshire. TreeView supports displaying trees in different styles and can show additional information such as edge lengths and internal node labels. Users can rearrange terminal taxa, reroot trees, and view multiple trees in sequence. The program also allows saving trees in various formats for use between different programs. TreeView is available on the web at http://taxonomy.zoology.gla.ac.uk/rod/treeview.html. It is available for Motorola 68000 and PowerPC based Macintosh computers, and for 16 and 32-bit versions of MS Windows 3.1 and Windows 95/NT. The interface for both platforms is essentially the same. The program was motivated by Mike Charleston, and the author thanks users who reported bugs, made suggestions, and tested trial versions. TreeView provides a versatile tool for viewing and manipulating phylogenetic trees across different operating systems.TreeView is a simple and user-friendly application for viewing phylogenetic trees on personal computers, compatible with both MacOS and Windows. Unlike some phylogeny programs that require data sets to be loaded before trees can be displayed, TreeView allows users to view trees directly. It runs as a native application on both operating systems, supports standard fonts, printers, and native graphics formats (PICT and Windows metafile), and enables drag-and-drop file operations. TreeView can read a variety of tree file formats, including those from CLUSTAL W, COMPONENT, fastDNAml, Hennig86, MacClade, PAUP, PHYLIP, and the Ribosomal Database Project. These formats use a variant of the nested parentheses format, known as Newick or New Hampshire. TreeView supports displaying trees in different styles and can show additional information such as edge lengths and internal node labels. Users can rearrange terminal taxa, reroot trees, and view multiple trees in sequence. The program also allows saving trees in various formats for use between different programs. TreeView is available on the web at http://taxonomy.zoology.gla.ac.uk/rod/treeview.html. It is available for Motorola 68000 and PowerPC based Macintosh computers, and for 16 and 32-bit versions of MS Windows 3.1 and Windows 95/NT. The interface for both platforms is essentially the same. The program was motivated by Mike Charleston, and the author thanks users who reported bugs, made suggestions, and tested trial versions. TreeView provides a versatile tool for viewing and manipulating phylogenetic trees across different operating systems.